Not directly possible in RSMB.
And it's on our feature request list.
Ideas of some imperfect workarounds.
If you use Maya or Softimage and Mental Ray + AE there is a different option which is in SmoothKit (SmoothKit Directional that is part of the SmoothKit plugin set) that allows blur one way only. Probably still works better to do a bit of RSMB first if you do SmoothKit Directional One Way. If you already have the MV, and also have our RE:Map plugins, there is a way to convert the Motion Vectors to a form SmoothKit understands with RE:map Displace. I can go over with you at techsupport if you want as it's a bit convoluted. The only two shaders I know are LaMaison Mental Shader that has this as a target option and Mental Images mip motion vector shader where for that you have to set boolean "blue_is_magnitude"... (read the doc).
Another way is to apply SmoothKit Frame Accumulate after RSMB with 1 for Previous frames and make the Blur Amount 1.0 in RSMB. This will give you a long follow trail. This only works if the RSMB blur amount is over 1.0 (so the motion blur trail connect to the previous frame)
I tried before playing in Maya with Shutter Open to render the Motion Vectors time offset from beauty pass but the result is not always glorious.