From what I know, you're supposed to plant comfrey like you mean it. It's gonna be there a long time and if you touch it, it spreads.
Borage is in the same family, is not perennial but reseeds if you want it, and (i suppose) can be crowded out. Might be a better option. Probably has some of the same properties.
Another option is clover.
http://ohioline.osu.edu/sag-fact/pdf/0009.pdf Another, more traditional cover crop guide:
http://mysare.sare.org/publications/covercrops/covercrops.pdf I think the best thing is to be diverse in the choice of plants. One plant can't do everything for you, just like there is no magic bullet.
Buckwheat is nice. It depends on what your objectives are, I suppose.
William