The object removal plug-in (motion key) looks interesting. I haven't actually tried it yet. But I'm wondering if the reverse can be accomplished. Rather than removing a moving object from a static background, can you key out the static background and keep the moving object?
The BCC Motion Key filter is capable of removing foreground objects from a background but the filter has no way to isolate the foreground object and composite that on a separate background. It's something that has been discussed internally in the past though and we may offer that in a future revision of BCC.
Although Motion Key filter was not designed for this task you can try to apply a difference key with the original. I expect you may see problems with the edges but you can further refine the edges with a matte choker. In other words you'll have to experiment. Good Luck.