What's needed to put a PC fan into reverse?
Reversing the direction of rotation will be difficult.  Since you only supply power, there is a controller in the fan that senses rotor position and commutates the motor accordingly.  If you can get in there, you can probably reverse the direction by flipping two of the three sensors and two of the three windings.  That won't be easy to do since these are all nicely integrated onto a small board.

Supplying negative power won't work, just fry the electronics.

However, you can still "reverse" the fan by simply installing it backwards.