A OpenRC script should be fairly simple, as should a script for the modern van Smoorenburg rc in Debian. They are both largely just scripts of variable=value assignments.
The service file for systemd looks pretty simple, so that's probably not a problem to port to some other init system.
But what's in the "nearly" above, do you have some other concrete requirements for the distro? Any technical reason very small busybox-based systems would be unable to support your agent?
Deviceplane sounds like a very good idea for a niche that really needs something like it!
Are there any plans in the future to support more linux distributions?