I did get experimental multiplayer to work in https://hiina.itch.io/mushi-lantern (which also happens to use a gpu-driven cellular automata you manipulate to indirectly affect things). It does authoritative multiplayer; clients still run their own simulation but the host sends any changed state over in snapshots, which get lerped to on the other clients. So even if there is slight nondeterminism from floating point or timing or whatever, the clients converge. For this sort indirect/imprecise manipulation, it seems to work well enough you don't notice the lerp.