I don't know that the X500 will fit a 1200 motherboard. The 1200 motherboard is a little bigger than some of the new motherboards and has other requirements (keyboard?). I'm surprised Loriano didn't plan for a 1200 from the beginning as I would expect this would be a good market opportunity for users like you who want room, a CD-ROM, and internal power supply for their 1200. The X500 does support a picoPSU up to 160W which is plenty for an accelerator with all the fixings. I imagine that some customization of the case (back plate?) and connector extenders would be necessary to make this a reality but then Loriano has been accommodating and communicative so far. You should try contacting him at least. Maybe you could report back what you find as I'm sure there are other 1200 owners that would be interested in doing the same modification.
Oh! What a shame

It would have being the perfect solution for my A1200. It would clean up the mess in my room big time and allow me to do all sorts of upgrades, even including adding additional 32 Mb of SIM RAM for my Apollo pushing it to 64 MB of RAM without effecting my keyboard in any way. It would be like exactly towering my A1200 but not really towering it in a PC tower but in another A500 like keyboard case. I will try contacting him as you suggested, any idea what is his email address?