Would it be possible, through plugins I assume, to have functioning players all the way to player 11 or 12?

I doubt it. There will be lots of stuff hardcoded with the assumption that P9-11 are unused, and you'd need to track them all down and fix them. So sure, plugins might exist that did this, but it would probably be a lot of work and wouldn't be a simple case of flipping a variable from false to true or anything.

For example, the All Players group includes P9-12, but when you use the Create Unit trigger action it will not create units for P9-12 (ever, either in All Players or for a specific player); to do this you have to create it for a P1-8 player and then use a Give action to give it to P9-12.

Clearly they've specifically disabled Create Unit working if the specified player is P9-12. So that's just one of the many cases that would need to be patched for full functionality for P9-11.

Not to mention Player 12 is also reserved exclusively for ownership of critters, vespene geysers, mineral fields, and anything else that's "neutral".