I have been playing with a similar device, the
Trango Cinch. I have not come across anyone else using it in tree climbing, but it seems quite popular with the rock climbing guys.
It looks like it works just like the ART positioner, although I have never had my hands on one to compare.
The cinch is not at all limited to a flip-line, it is quite adequate as a replacement for a belay device or friction hitch, either SRT or DRT. I was attempting to use it for SRT, but that seems a pinch impractical if using it with foot ascenders. It will not allow any movement if the rope beneath it is loaded.
On the other hand, while it doesn't quite self-tend, it pulls up very lightly almost like a VT with tending pulley and it lowers you much smoother and controllably than any friction hitch I have ever tried. I'm pretty certain it would work great as a flip line belay device.
Can you install the ART positioner mid-line? It looks like that might be difficult or impossible. Not so with the Cinch.
This video gives you decent idea of how it works, but the guy is testing it on 9mm rope. Too small, I think, for tree work. I'm pretty sure a big 1/2" climbing line would be quite a tight fit, but my Velocity rope (11mm) works great in it.
another review: