quite right tying 2 +12 volt lines will just increase amperage and using a split rail to make a 24v difference (-12 and + 12) can be done, but yes you will need to regulate it as it will float and you dont want to send negative voltage into any logic system that is not prepared for it , and I dont know how a switching supply will act over time

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Exactly. If you have a linear supply with separate regulators then I am sure this works fine. But these are switching supplies, and work very differently! I would strongly advise against trying this with a found switching PSU.

What you probably can do is tap DC right off of the transformer output and hang your own regulator there, maybe a small LM317 circuit or such. Assuming that the output has a few volts headroom.

I will probably by modifying an ATX supply for my own future MDD recasing project. IMO it should only effect the resale value if it is or looks sloppy. Do it need with some heat-shrink tubing and it should be fine. The prices are dropping so fast on G4s now that you probably wouldn't get your money back anyway. You will just need to enjoy your prize!