Honestly, I would plant the beans.  Spray when alfalfa has regrown 8-12". Add some residual. I would use max rates of Roundup/Glyphosate. Twice.  Maybe toss in some Classic on one of the applications, or Synchrony.  I think either Classic or Synchrony might help the Glyphosate.  But the Glyphosate is going to be the mainstay of what will work on the alfalfa.  I won't guarantee the Glyphosate will wipe out the alfalfa, but I think it will set it back enough that the beans can get ahead.  

That is how I think I would look at it, if I were to plant beans.  But again......  I would probably plant corn. :)