I opened the thing up today to clear out the bits rattling around inside, and noticed that the connection between the power supply and the msx hardware itself is relatively simple; 4 wires (red, yellow, blue, and black) carrying DC power and ground.

Can anyone tell me what each wire is supposed to carry? It might be easier for me to use an old PC power supply.

is your computer is this list of service manuals ?
http://www.msxarchive.nl/pub/msx/mirrors/hanso/service_manuals/

edit: found it, but it's really hard to read and I can't tell which wire is what color.

that's why I still use my good old openMSX emulator... just as good as the real thing for my purposes. But I wouldn't pass on an OCM (if it became available)

Or since shortly openMSX on the Wii

then you probably need a voltage tester.
probably red is +5v , yellow +12v , black is ground and blue ... -5v ?

from what I can see, one is +12v, one is -12v, one is +5v, and one is ground. also, a voltage tester won't do me any good as I have no way to get power to the thing at the moment.

edit: i may have figured it out. I think red is +12v, blue is +5v, black is ground, and yellow is -12v. I'll sleep on it tonight, and look at it again in the morning.

Wow, that one is broken in an ugly way...
a) yes you did something foolish, check here before you buy stuff next time ;-)
b) not all hope is lost!
c) good luck...