I can only suggest you to check default configurations, suggested by HP - whether or not they have options with like 5 GPUs 250-300W each. If they do have such options, you should be able to create cables yourself (Soldering is the most simple part here).
But you might also run into limitations, related to GPU physical dimensions, power connector location and etc. Without trial&error it would be really difficult to tell whether or not they would actually fit.
I doubt I would be able to add anything useful via PM.