[Dave LaRonde] “….and the elaborate process you describe is also a good argument for using a chroma key background to get the shot, and why chroma key exists in the first place.”
That’s also why it’s critical to know what you’re doing when setting up a chroma key shoot in the first place, to make sure that you don’t spend hours in post trying to pull a key from a crappy source plate. If only I had a dollar for every clueless low-budget producer who thinks that all you need to do is throw a green sheet on a wall, stick talent in front of it and shoot it with a DV camcorder…
Anything can be rotoscoped by a competent compositor. As you correctly state, the whole point of doing it right to begin with is to avoid doing roto. It sounds like the O.P. didn’t have the luxury of being present for preproduction meetings and now has to extract the talent from a white background that may never have been intended to be removed in the first place.
Any way you slice it, there’s no magic bullet to do this shot without at least some roto or garbage matting and/or combining multiple keys. Fortunately, Motion 3 now has spline-based roto masking tools.
Good luck!