Yes, OpenBSD/arm64 runs on M1/M2 machines.

I understand M3 and M4 are (by some accounts gratuitously) different and harder to support. Do you know of any future plans toward those?

Unfortunately no, I don't. At the very least we'll likely need to wait for support in Asahi Linux which provides for us the m1n1 bootloader, device trees, etc., as a first-stage.