the problem I see with the gang valve idea, is DIY doesn't produce much pressure, so I see it easily taking the path of least resistance, which is gonna be the open valve with no tubing that's submerged.
it may work though.
however, I don't see a reason to regulate DIY. Often you're lucky to get consistently good/adequate CO2 rates to begin with. If you over 10-15ppm, big deal, the fish won't care, and the algae will hate it.