A PCMCIA RAM card needs to be prepped as a disk (using PrepCard) before you can boot from it, but otherwise it should work on a stock system. It'll need to have a working battery, of course. Mine doesn't, so it's always recognized as RAM from a cold boot.
I think CC0: tops out with a 4MB card, which may not be enough for a full 3.1 installation (plenty of room for a cut-down install, though). If you need a PCMCIA bootable solution, a workaround that I used for a while was to mount CF0: from the startup-sequence of a bootable floppy (see cfd.lha on Aminet) and then to reassign the system to the CF card. Not truly bootable, but good enough.
