Sure, no problem! Note that 3.9 hasn't been released yet, even as beta. Your animator is going to love it.

Just encountered this... because I want events to trigger, this form of reversing wouldn't work. Also, grabbing all the keys in the editor and dragging them in reverse order doesn't seem to work for me - maybe because ik or constraints don't work the same when you reverse the keys? My character's feet are going through the floor and the animated image regions are twitching at the loop point. It's hard to tell what exactly is off. Edit: It's at least partially becasue if you keyed the "mix" parameter on the first frame, this is now on the last frame, so the ik and constraints only apply for 1 frame.

I was thinking you could sidestep this if there was an option "bake" animations, or at least "bake" them in reverse - basically have it do whatever the "play backward" button does in the editor and key every frame (but also including events in this). So at least it would be easier to make a reverse animation in the editor.

Another idea I had was if it could have an option to export anims in reverse, with "_Reverse" added onto the end.