I think theres a scripts somewhere that you can use, but I'll tell you the wacky work around:
take your motiontrail and create a new nurbs curves starting at the motiontrails start using point snapping.
click a couple more times anywhere to complete the curve. Parent the curve to what ever created the motion curve. then "Create an Animated Sweep" out of that curve, select the edge isoparm on your newly created nurbs surface and then duplicated surface curve. That should give you the curve you're looking for.