If this thing does take off you could always talk to AmigaKit to see if they are interested in funding a drop-in A1200 MB replacement option 
That would be awesome, and I think you could easily make one to fit both A600 and A1200 cases. Regardless, this is something I would try to do myself. And yes, I could always use keyrah, but I'm not sure the USB on the expansion will handle USB keyboard/mouse well enough so that PS2 is not needed at all. I must admit that personally I would rather see the PS2 ports replaced with USB-1.1 ports alltogether, but I just assume PS2 is much cheaper and simpler to implement.
If you did go for the replacement 1200 MB option I wonder how much work it would be include a CardBus slot (eg update the PCMCIA)?
I think I'd rather put a USB card reader there, frankly, it's been years and years since I last saw a cardbus card now.
