Looking through a stack of old PSUs , I see that -5 is very low current. -12 is slightly more but still very low.
I'd guess that the '-' supplies are used for bias of some sort or other very low current applications
- I don't think I've ever seen logic/drives or other hardware actually powered from a '-' supply.
Given that PC PSUs are designed to support "worst case" (fully loaded system) - I'd guess that you
could just regulate -12 down(up?) to -5 .
You should of course check the actual current draw on -12 to be sure this doesn't overstress it.
Dave