I recently saw the floating shelf concept shown in the attached picture. It appears that about 1"-2" of one edge of a live edge slab was ripped to serve as the wall-mount. It also appears that dominos were glued into the mounting strip with the remaining part of the dominos inserted into the rest of the shelf. I do own a Domino 700XL and I assume that 14mm dominos would be the appropriate size. Based on the XL manual the deepest mortise possible is 70 mm. I envision shelves that are maybe a total of 10" (254mm)deep So, my questions . . .

1. Is there a way of making a deeper 14mm mortise with the XL; up to 100mm deep? The ideal would be to be able to glue about 40 mm (just over 1 1/2") (of a 140 mm domino into the mounting strip and the remaining 100mm (almost 4") into hanging depth of the shelf. The shelves may be 5' long so I could probably use 5 dominos on each shelf (although I might not need that many if I can go into the shelf 100mm).

2. If I can't easily make a deeper mortise with the XL, then my question becomes whether a 70mm (2 3/4") deep mortise into the shelf would be sufficient to hold a live edge slab approximately 2" thick and 5' long?  Again, I can use 5 dominos over the 5" length. The majority of the weight would be the shelf itself as we don't plan to put anything real heavy on it.

1. Yes, You can make the 70mm deeper by using a drilling jig/guide bushing and a drill bit of the sane diameter. I've done something similar with my DF500.

2. I'd prefer deeper tenons (oriented vertically or double tenons).

Why not just use the metal mounts designed for floating shelves?

Thick timber cantilevered off some thin dominos just seems to be asking for trouble, even if technically adequate?

CRG has got some good points there.

But if I were mounting those shelves, I'd get a construction lumber of similar size from a big box store (pretty cheap, about $15 - $18), and do a test run (add weight as necessary). If the mock-up shelf fails, I know where the weakest link is.

Testing for sure.

I could use the metal floating shelf brackets from Rockler. However, they depend on being able to drill a 3" plus hole into the wall studs. I can certainly get a bit long enough. However, ensuring that these holes are exactly 90 degrees to the wall surface seems a little iffy. I'm sure there are ways to do this, but it just seems like, if there is even a very small difference off 90 degrees, especially if the error is downward, problems will result.

The dominos are assured to be straight in both pieces of wood. Drilling a pilot hole into the wall for screws to mount the back piece to the wall doesn't seem to require quite the same level of accuracy and precision. Even very close should get the piece attached to the wall mounted flat.

Plus, after talking the whole thing over with my wife, we decided to just not do the shelves. Enough oak, maple, or cherry to do three 5' long shelves would require 30 board feet of wood; either 8/4 live edge slabs or boards or 4/4 glued together. At today's prices with my current access to good quality lumber that could be $600+. It doesn't seem worth it. Add $20 apiece for maybe up to 9 metal brackets or the cost of a 14mm Domino cutter plus the cost of 14 mm dominos (I don't own a 14 mm bit or dominos.) the cost isn't worth it. We just decided to find other ways to fill a blank wall that was created by a remodel; maybe with  a nice sofa table size table below and artwork on the wall. I am a turner and have more bowls than I know what to do with and my wife is a polymer clay artist (actually a multi-talented artist) who has a number of wall hangings and other art.

Using metal rods, of course, is the common way people employ for such slab shelves, but it's missing the point in this case

If the OP is like me, I'd like to use my Domino machine as much as I can, and explore its use in many other applications where possible. I enjoy the challenge and learning process, and draw satisfaction from the outcome if it's a successful one. And that's how I grow as a woodworker.

I definitely would study and explore the use of dominoes (if I have a DF700) for that kind of shelf, failing which I'd then resort to the use of rods. I find deepening dominoes with a long drill bit pretty easy (which I've shown elsewhere in the forum)..

Using metal rods, of course, is the common way people employ for such slab shelves, but it's missing the point in this case

If the OP is like me, I'd like to use my Domino machine as much as I can, and explore its use in many other applications where possible.

I think everyone prefers to use the equipment they own and are familiar with.  However, if you accept that 2” into the studs is the minimum that will give adequate support, then you need 2-5/8” depth of cut.  Will the Domino machine mortise that deep?  Are you going to glue into the wall?  Then the bottom edge of the shelf levers into the sheet rock.  Is the Sheetrock surface hard enough to sustain that pressure? 

The tenon will be sustaining loads in both shear (which it can probably deal with) and bending (for which I am suspicious).

I don’t think the domino machine is the right one for this.  I have a variety of doweling jigs and a biscuit joiner.  I suspect that I could hang a shelf with no load using either.  I also suspect that it would fail quickly.

Just because you have certain equipment, does not mean that you should use it.   

Snip.
I think everyone prefers to use the equipment they own and are familiar with.  However, if you accept that 2” into the studs is the minimum that will give adequate support, then you need 2-5/8” depth of cut.  Will the Domino machine mortise that deep? 

snip.

Just because you have certain equipment, does not mean that you should use it.

The depth of a mortise is not limited by the cutting depth of the DF700, or for that matter, the DF500. I've used my DF500 like a DF700 (to an extent) by deepening the mortises milled by the DF500. Sometimes, I use the 10mm dominoes made for the DF700, sometimes I make my own to suit the length. Others have done that, too. So the same applies to the DF700.

This isn't a case of "if all you have is a hammer, everything looks like a nail." I'm talking about EXPLORING expanding the capability of the DF700 or DF500 to handle applications that are beyond the imagination or intention of the designer of the machine.

But safety first. That was why I suggested testing it out with a cheap lumber. In some of my past projects when I had doubt about the joinery strength (whether it was loose tenons or other joinery), or tried to locate the weakest point, I used mock-ups where needed.

Everyone learns differently. I find learning outside the box just as satisfying as finishing a project to my own standards.

@luvmytoolz I have built a bunch of them like that over the years. Usually just plywood strips as the frame and 1/4" MDF skins. Then they get veneered or laminated with HPL.
I've done the solid wood version a few times, but drilling the holes is more of a pain than building pockets into them. They turn out far lighter that way too, then the brackets are not supporting as much shelf, leaving more strength for stuff