TrueNAS actually implemented a sort of protection mechanism into their version of samba that will take a snapshot of the time machine dataset after a time machine client disconnects - the idea being that once the client is disconnected it should be left as a usable restore point. An ix employee (the apparent author) comments in a forum post that they never documented it very well, but their work could probably be up-streamed.
That work hasn’t been up-streamed, to my knowledge, and I never found a comparable feature set elsewhere.