No, the cut thickness limitation on an SCMS is that the sliding head can't pass over the workpiece because of interference with the top of the workpiece, so those are hard limts.

The SCMS is a highly specialized tool developed primarily for length cutoffs of construction stock and moldings with precise control of both the cut and bevel angles.  Its design inherently limits material thickness capacity as both the sliding housing and the blade axis must clear the top of the material being cut.  For cutting thicker stock with similar capabilities a large capacity radial arm saw with a 14" to 16" blade would be one option.

Would a jigsaw work? I have a great handsaw that I use for really thick cuts. Good aerobic exercise.

Why don't you take a sample of what you want to cut to your nearest Festool showroom and give it a try for yourself?

I remember doing it years ago, I don't remember the size (if it was 120x120 or 145x145) I do remember we had to slide it out from underneath the sawhead to flip it. (I just checked if I had any cutoffs lying around to test it for you, but I can't find any)

This would be the main concern for me.

please be sure to not have any flex in the lumber that could pinch the saw blade. Take the cut slowly and feel/hear the saw starting to bog down. That usually precedes a kick back.

Sure, the cut you describe is theoretically possible, though you may have to remove the dust shroud first.  But if the timber you're working with is not already perfectly straight, it will not result in a square end, and more critically (particularly if you're cutting more than just a few inches off the end) it will start to bind as you make the cut and possibly kick back something awful.

If you still want to try, make sure to lay down the timber so that any bowing is positioned relative to the fence (along the y axis), rather than relative to the saw bed (along the z axis).  Do an initial two-step cut a mm or two proud of the final length, and then do a finishing cut to remove the final mm so that you can cancel out the deflection that was created by the bowing. 

And engaging the hold down clamp would also be highly recommended.

I am not a lawyer, but if you alter the Kapex (remove the blade cover) and get injured or burn up the motor, you may not have recourse through Festool.

I fail to see how cutting 120x120 in two passes by flipping is any more dangerous than cutting a single 120x60. Exact same precautions would apply to prevent pinching, kickback etc. And why would you need to remove blade guard? It's spring loaded and opens all the way to the arbor. Kapex cuts 120mm tall molding, so the shroud should clear it. If needed, rubber dust shroud can be rolled back without removal, just like Festool shows in a training video.
Important to set the depth stop to 60 mm like you do trenching or lap joint. Then cut, flip, cut. No excessive stress on the tool that I can see.

If I decided to do this cut using my Kapex, I’d do the first side going 1/4th through. Then,  widen the kerf by making a second cut going 1/4th through. I’d then cut the 1/2 thickness making 2 passes. I’d flip the board and repeat the 2 cut technique. Of course the very last cut goes all the way through.

The idea is to reduce the chance the blade will bind and kick back by creating a wide kerf.

 I guess one could do the cut in thirds, but by being aware of resistance while cutting one can back off before a kick back.

I use a lot of highly figured hardwoods. If I’m cutting through a board thicker than 3/4”, I’ll make the cut in two passes. I’ve seen wood move a lot while making a cut due to internal stresses changing. It doesn’t take much movement to bind a blade.

You should be able to cut a 6"x6" post, without modifying anything. Accuracy will be lost at some point tho.