So a compact air-portable external GCU that we'll only use externally and on the ground (so no particularly certification issues) combined with the APU running the aircraft a/c is what we'd like to do. GPUs to run the kit on board is at-least quite easy to rent wherever we are.
May be that by splitting the a/c unit from the power supply you will have a better chance of finding off the shelf solutions.
More likely to fit in avaialable space and also might be possible to leave a/c unit in cargo hold while in use.
If you google on "emergency air conditioning" or "industrial portable a/c" or similar you should find leads to manufacturers that can solve the a/c part of the problem.
Once came very close to having to rent one in the middle of winter to cool multiple large computers until we got approval to open a window.
(The window of course was not -designed- to be opened...)
Appropriate generators should be easier to find or power can be from ground sources.