I just figured out how to use the BorisFX "motion key". I've searched, but can't seem to find a way to invert it. That is, take out the background and while keeping the moving object there, also adding a background. A bit hard to explain, but something like; there's a car going down a street, I want to take the car and put it in front of a different background.
I understand the effect that you are trying to create (isolating a moving object and removing the background) and while this is not the problem that we were trying to solve with the Motion Key effect and therefore it is not an easy thing to do within the filter, it may be possible to achieve with some compositing tricks.
I think it's always good to experiment with the filters and try to use them in different ways - and in the case of the effect that you are trying to generate you might want to run the motion key, remove the foreground object, then duplicate the clip and composite the clip to which the motion key was applied in a difference mode over the unfiltered original. Then you can use this as the basis for a holdout matte that can be tweaked and refined and then used to isolate the foreground object and remove it from the background. Although this may seem like a lot of work, I'm sure it's a lot less work than rotoscoping. :)