I was reading in the last issue of creative cow magazine that a time saver in AE is to render out an image sequence and bring that in as the top layer. That way, you only need to render those parts of your project that are changed. Is this possible in motion as well?
Sure, you can export stuff in your timeline. Just make a selection and choose Export Selection from the File menu. In the export dialog that appears, select 'Import into project' from the After Export popup.
There's no particular reason you should be exporting an image sequence, though. You just need to make sure you export using a format that is high quality and supports an alpha channel. I would recommend a Quicktime (Animation codec) movie or TIFF or PNG image sequence.