In a similar situation, I was thinking about upgrading my 65W A500 power supply the other day when I noticed an amazing similarity in size between it (the hollow A500 type PSW) and a uATX power supply. Those little guys give you between 200 and 300W!
I don't know if they will supply -12V but a small DC-to-DC converter or even a linear regulator should give you the voltage needed for the Amiga's audio ICs. You only need ~100mA for -12V anyway.
It looks like it is possible to shoehorn a uATX PSW into the A500/A1200 PSW case. :-D
Unfortunately when I picked up a used uATX supply the other day it was dead (bought at a surplus store) :-(
