One of many good point of a bootable PCMCIA slot is you can boot direct from OS3.1 or OS3.9 if a accelerator is present or even 4.x with small changes for compact flash. You maybe can even use part of the compact flash as fast ram. I don't know if the above is possible but its worth a look.
If you wanted to throw everything in you could add sockets for FastATA chips as well,so you could end up with a single board that flash the roms,support 4x clockport,support FastATA and anything their care to add via a single board.
ALL MODIFICATION AT YOUR OWN RISK AND I AM NOT RESPONSIBLE FOR WHAT USER(S) DO.