It looks like I have it worked out, in a couple of ways.
I got a 5" pad with direct 14mm threading. The height is almost exactly the same, within a mm or 2.
But, because of the diameter difference, it does rub. I'll just make a spacer to keep it from going so deep, easy. I didn't recall seeing it in the description of the pad, but it came with 50 pieces of 100 grit abrasive and the little 1/4" shank arbor/adaptor came with it too.

I also found a flange mounting arbor that is direct threaded, converting to 5/8-11. The two flanges and nuts, plus the pin-spanner, were separate. With this I will be able to use flap wheels, grinding wheels, or even cup wheels (wire)

$43 for all of it.

No, it's not going to work with the brush. Any increase in diameter will take that out of the picture immediately. I have never really embraced the dust collection aspect of the RAS in the first place. The brush takes the "feel" out of it. I generally use the RAS pretty aggressively, with coarse abrasive, as a smoothing/blending tool. To use it "flat" is nearly impossible with the brush in place, so I removed it, long ago. Then when I found out that it is virtually irreplaceable? Why bother.
Also, when used in a tipped fashion, the dust collection is pretty ineffective anyway, and once you get the offset of a flap-wheel in the mix, the brush is going to be so far away as to be useless.
I'm just looking to increase the effectiveness of the machine by adding to the variety of discs it can use.
The original Festool backing pad has the proprietary hooks, which is a limitation on it's own, this gets me into the more common hook and loop.
The thread adapting arbor and flanges was just a bonus that came up in the investigation.

I've been fiddling with a Pat Warner style router sled/fence setup to make milling down precise aluminum blanks easier. Some years ago I bought Pat's PDF's, at the time he also offered one-on-one tutoring in his shop. I regret not making the trip out to meet him personally, it would have been a hoot. Now I'm stumbling through the learning process somewhat blindly.

Pat's setup used a fence that fully encapsulated the bit and a sled that rode along the fence with the material extending slightly past the sled's edge. The main idea is there is always 100% contact between the fence and sled, and the material is secured to the sled in a death grip.



This version had its maiden run today, the edge finish left a lot to be desired, but with a couple of minutes of clean up the result was bang-on parallel.



Next up is a micro-adjuster and precision stop so I can dial in the fence offset for different bits. Any recommendations on this would be appreciated. My inclination is to have an adjustable stop that gets set so there is a 10mm offset to the bit and locked in. Then the fence is adjusted out as far as necessary for whatever rough thickness I'm starting with, moving back ~1mm for each cut until it reaches the fixed stop.

Unfortunately Ophelia is preventing us from doing any yard work... guess I'll just have to spend the weekend in the shop. 

Thanks,

RMW