The red cropping tool in compressor is just the kind of thing I was looking for, as it allows one to adjust the cropping values visually over the image rather than typing them in. However, the rest of the export, setup and render process seems to take noticeably longer than doing it from within Motion by resizing the canvas. One can only hope that a future edition of Motion will incorporate such a useful visual cropping tool.
I have converted the HDV to and exported with ProRes 4444 and the keyed edges look much smoother.
Thanks very much for your suggestions Andy, I like to learn of the different options available.