I’m not too knowledgeable about SELinux, but Podman itself can create unsafe (privileged, etc.) containers, so I don’t think naively restricting user namespaces to Podman is going to help, exploits could just create the namespaces through Podman instead of doing it themselves.

BTW, if I remember correctly Podman isn’t setuid root except some very small helper binaries, almost all of the process of creating the container can be done without root privileges.