On-demand daemon activation without sockets?

Systemd’s on-demand socket activation has been very useful for me, to launch daemons as their own users and with privileges appropriate to the daemon only as needed rather than at boot where they just hang waiting for input.

But many times, it’s very awkward to route the call through a socket. Depending on the particular daemon and what its use is, that’s not always the most “natural” way

Is there an alternative (either within systemd or outside of it) which would enable a sysadmin or package developer to configure a daemon to:

  1. Start up on demand, only as its needed or called,

  2. Start up as its own user with its own privileges,

But without using sockets / network-based activation?