Since religions are human inventions, I would guess that any comprehensive simulation program already produces all conceivable religions.

But I'm guessing that you meant to talk about the simulation of all conceivable gods. That is another matter entirely. Even with unlimited computronium, you can only simulate possible gods - gods not entailing any logical contradictions. There may not be any such gods.

This doesn't affect Tipler's argument though. Tipler does not postulate God as simulated. Tipler postulates God as the simulator.