I understand that the power switch is no more than a shorting link. You could try tracing where the wires that go to the switch arrive at the psu and just link (short with a paper clip?) them together. Do this close enough to be sure that there is no possibility of there being a fault between the point that you link and the actual psu. I also understand that there is only a few volts on these contacts so you don't need to worry about serious insulation here, just tape it up to stop it touching anything.
Rans6.....