You can shoot projects for companies that aren’t represented by the AMPTP. For example. A24 isn’t represented by them, so they can still shoot and will likely have received a waiver from the unions to do whatever they need to do, same with low budget/actual indie films.
No studio is going to buy scab screenplays for one main reason: they will not even look at a script that isn’t submitted by an agent, manager, entertainment lawyer and no one worth their salt is going to jeopardize their career to do that. The strike won’t last long enough to go through the development process anyway, so they’d dump it when a deal was made