i was going to do something similar of my own. i was going an ATX PSU and put a switch on the purple wire to ground. it should work and an ATX PSU will give you a lot more flexibility on wattage output. also you wouldn't need to hook up any sort of anything inside besides what you need.

hope this helps. here is the thread to my post about this. PSU question for ext. FW enclosure

by the way i like the idea of using a g4 case. i was thinking of using a g4 Cube's case at one point. maybe i'll put my Biege G3 tower to work as a RAID tower. also you might think about doing a LED HDD activity light mod for each of the HDD. here is a good link to use.

good luck

2.4gHz 15" MacBook Pro, 1.66gHz Core Duo Mac Mini, 2.5gHz G5 QUAD, 733mHz Quicksilver, 450mHz G4 Cube, 700mHz G3 iBook, 350mHz Sawtooth G4, 350mHz Revs. A and B B&W G3, 16mHz Powerbook 100, 8mHz Macitosh Classic.