Hmmm. You have Pro Pack, right? You could place each frame on a separate rectangle, and keyframe the transparency values to maximum on the unwanted frames.

I don't know if you saw these earlier; I documented my experiment in Poser reflections over in the 3D/2D Animation forum. Maybe you'll find it useful.
Less boring animation test -- Poser reflections
Poser reflections -- Take 2
Poser Reflections -- The Final Cut
Poser Reflections -- Special Edition