Good luck making it work with layers. Attempting to paste keyframes into a layer can make Poser crash. You can't apply "Resample Key Frames" or "Retime Animation" independently of the base either. I'd be happy with a script that just reverses the selected keyframes on the base, and Collapse All first if needed.

Trying to reverse a layer that's in add mode might be asking too much, unless there are already keyframes at the beginning and end of the selection to be reversed. For example, reversing a selection from this layered animation would require adding keyframes to the layer, and that would change the spline.

In the left screenshot, the base layer is selected. In the right screenshot, layer 1 is selected. The black line is the two layers added together.