The Firewire idea is an interesting one. A FW bus is supposed to supply 45W, although I don't know if it's per bus or per port. In any case, a Firewire "hub" is a bit unusual, as daisy chaining is the normal way to connect multiple devices. With that said, my rackmount G4 does have a 4 port FW400 hub in the front of it.
If it's designed to supply 45W through each FW port, that would account for a whole lot of extra power...
I've never used one of the Al displays, so can't comment on the brightness. Interestingly, in my reading, I've seen many comments that reference them "failing" due to increased current draw of the aging backlight that's more than the 90W PSU can supply.
90W at 24V is 3.75A. It wouldn't be terribly difficult to built a 24V PSU that can supply 5A or so...perhaps a project for one of these days.