Isn't the issue in this case caused not by suid, but by a daemon running as root reading files from a tmp dir? Seems like a socket-activated daemon wouldn't solve this specific case.
systemd-homed stores most of the user specific information in the home directory `~/.identity`, but since the file contents have to be signed the changes need to be done though a daemon, which is talked to via IPC (homectl does the talking to systemd-homed).