Sorry to be pollyannish, but I'd say any messing with the Prius computer beyond factory options is a really bad idea. Even cautious bit flipping in an unfamiliar environment could have unintended consequences. Especially with a jury rigged setup, which sounds like the only possible option becasue Toyota has a propriatary controller.
Hypothetically, if someone messed with their car computer and it went out of control and killed sombody, I think it would qualify as manslaughter. Even a distracting mod might qualify if the accident was borderline negligence. Just saying... bad idea.