Adjusting derailleur with bike upside down, is it ok?
Turning it upside down seems to make the most sense, assuming it can be easily balanced. Derailleurs work by spring and cable tension, not gravity, so there should be no problem with working on it with the bike upside down. The only issue you're likely to have is reaching the shifters (assuming they're bar-mounted). Also, if your bike has disc brakes, you should check out this post about potential risks of inverting the hydraulics.

I pretty routinely will invert my bike when working on it, as I also don't have a workstand, and it's never caused problems before.