Couldn't you theoretically add liquid cooling to the existing heatsinks? It wouldn't be as good as liquid cooling directly, but the colder the heatsinks are the colder the processors are, which would let the fans run slower or the processors to run hotter.
It'd be a lot of work for limited gain though; a more useful solution might be to just get air-conditioning, either for the room (potentially cool several machines) or just get a portable unit and have it push cold air at the computer. Obviously it cools the whole room, and the computer is going to make the air conditioning unit work harder than it needs to, but the colder the air is coming in, the more effectively it cools. Plus if you get a new machine you don't have to transfer over the liquid cooling or buy a new liquid cooling unit, and if you get especially hot weather then you're not going to mind having some spare air-conditioning
If I had a spare room where I could do something like that then it's what I'd do; namely move my computers into a discrete air-conditioned room out of the way, and either setup another machine as a dumb-terminal or see if I can video cables etc. that will work over long enough distances to route them into my various, normal temperature rooms. If you have a basement you could even partition off a part of it, provided it's dry enough.