Although the SCSI Compact Flash adaptors are a good idea, the price is silly. The manufacturer would sell loads of them and probably make more money on the whole if they were to lower the price.
Rant aside, there is another possible solution which I am currently investigating. There were PCMCIA expansion modules made for the 500 series PowerBook that fit into the left battery compartment and connect via a PDS slot. I have one of these modules and currently I'm trying to get my 520c to regognize and boot off Compact Flash cards plugged into the slots via an adaptor. With my small selection of cards, the only ones that seem to work are the 32MB standard speed cards, which will boot the PowerBook just fine. I have also tried a 512MB card and two 1GB cards but they don't seem to work, causing freezes. I think this may be due to the speed of the cards: my 512MB and 1GB cards are all high-speed cards which may be causing issues with the 520c's interface. Or perhaps it is due to the larger capacity of the cards... More experimentation is in order because I would like my PowerBook 520c to run off a Compact Flash card of a useful size.