Just a head's up that I have a single-path export from the SynthEyes 3-D camera tracker to Motion running now. There's an example on the our website, see http://www.ssontech.com/download.htm, showing a 3-D point exported. The path continues correctly even after the point goes off screen, since it is calculated from a 3-D solution. For example, after a smoke effect's emitter goes off-screen, the smoke can still continue to billow out properly, on screen. The export is an XML file, you can drag it from the Motion file browser onto a layer (name).
Related comments, suggestions, requests, etc welcome - Russ