I tested version 17568 on the PTR and seems to be working okay.
My bar2 is missing though and I can't seem to enable it with /bar bar2 enabled on.
I would really love to get the HunterBar addon working for Bartender3. It seems like you are planning native support for this sort of thing.
Any way in which I can test this? I just want my Bar1 to swap with another bar as soon as the target comes into melee range. And ofcourse it should swap back to Bar1 as soon as they move out of melee range. If you want to point me in the right direction I'll try to figure it out.
I'll do some more testing and let you know of any bugs I find.