Exactly what I had in mind, thank you. If I hadn't already ordered something else in the meantime, I would definitely go with the MDT adapter and a proper fixed stock right now. I may get one anyway if the current option fails.
For lack of a better choice in the meantime since the original post, I ordered a plain Magpul MOE collapsible stock and the snap on MOE cheek rest. I don't know if it will hold up to the recoil, but I thought I may set the length and then fix it in place with adhesive. It's not like I ever move it anyway. The problem I have with some of the cheek rests is that when I'm comfortable on the stock, my cheek is halfway off the front edge of the thing. I have no reason to expect the Magpul MOE to be any different, but it's worth a shot I guess. Thicker buttpads may be an option to salvage the MOE if it fits wrong.
On the original FAB stock, I installed the optional rear monopod. The added inch or so, and collapsing the stock one position got me to where I wanted, but I still don't like the stock.
If I were mounting a fixed stock, I'd probably just mount a rifle buffer tube, but it's nice that the PRS can use either one.