To loop animations with Grease Pencil, I've been using the Time Offset modifier. This is works well when I want to restart the loop from the beginning but it doesn't do well with a "boomerang" type effect.
Say there are four frames that I'd like to repeat: A, B, C, D.
The Time Offset only (seems) to provide A -> B -> C -> D -> A -> B -> C -> D...
What I'd like to get is A -> B -> C -> D -> C -> B -> A -> B -> C -> D...
Is there a way to do this without manually copying frames?