A few months back I searched this forum and the rest of the Internet for the solution to this problem, and I couldn't find anything that actually worked properly, so I finally just had to figure it out for myself. I just remembered that I had meant to come back and post it here in case it will help anyone else avoid having to reinvent the same wheel.
The pictures below show the custom node I created called "Speed to RSMB", followed by where it connects into the Blender compositor, and then the settings of RSMB Pro Vectors using the resultant vector pass layer in After Effects.
Contents of Blender Speed to RSMB Node:
Blender overall compositor chain (I'm also outputting a depth pass here):
Use of resultant vector pass by RSMB Pro Vectors in AE:
Finally here are a few shots of the resultant render with and without RSMB enabled. These are from a video of an asteroid that is tumbling slowly forward along its longer axis, and doing a close fly-by of a stationary camera that is panning to follow it, causing motion blur both on the asteroid and the stars in the background, and finally the Earth that is unfortunately directly in the asteroid's path. :-) The asteroid, Earth and background were each rendered out as separate passes with their own vector and depth passes, but I deleted all of that out of the Blender compositor picture above to provide a cleaner view of how to connect the Speed to RSMB node.