the new version of Flame (v8.0) which is due out any day now will have selective motion blur per layer in Action that you can toggle on/off (as well as on/off for geometries).
if you can’t wait for it or won’t be upgrading you might try this for certain types of comps:
once you’ve got your comp like you want it go ahead and load up that Action node in Batch and make a bunch of copies and string them together. then, hide all but one layer in each Action node (remember to pipe the MatteOut for each layer). you can then turn on motion blur on only the Action nodes that matter and leave them off on the ones that don’t.
again, this only works for certain types of comps…but can save a bit of rendering time in the long run
good luck