Ok, but this all seems kinda over-the-top.
Can you please share your anim-graph montage slot definition / hookup information?
Regarding “it won’t magically know it should play the upper body slot montage, starting at the same frame the previous montage stopped.”
Actually, you aren’t far off - and I was totally missing that part.
Montages give you the power to easily do this since they offer a “current frame” value as output that would allow you to call in and start another montage at that same frame-time (note that it’s time, not a frame).
So more often than not - and make your own topic on it since it’s kinda Off Topic and specifically on the subject much more help can be provided - people who are developing (on this sorry excuse of an engine) bite the bullet and spend a few hours if not a few days building a custom Montage Wrapper function that is used specifically to extended the montage system in order to provide easily applicable transitions, curve reading, etc.
However -
What’s wrong with having the animation just be split?
Top stuff plays as montage on upper body so it’s constant, never interrupted.
Bottom stuff is a montage that automatically breaks to the regular animation when input is received (ex: stop montage on any directional input applied).
Wouldn’t that achieve the same end result with no extra bells / ABP complexity?