Agreed. The PCMCIA is essentially just a PDS module in the 5xx series, no different from any other. I would imagine the ROM is not programmed by default to search PDS modules for bootable partitions.

With the PCMCIA being an integrated part of every subsequent PB (until PCMCIA became irrelevant), it makes sense the ROM would be smart enough to search for a bootable partition on the PCMCIA bus.