I would expect some burning. 

I would expect you will see results within a week.  After a week, I would think metribuzin over top would be no different than metribuzin applied pre-emergence.  But that is my guess.

If you can posts pics, that would be interesting to see.

Metribuzin can be impregnated on dry fertilizer and spun on alfalfa up to 3" of new growth in the spring.

Metribuzin can be sprayed on dormant alfalfa in the fall, and before it breaks dormancy in the spring.

I had a farmer ask me once about spraying metribuzin on alfalfa in the spring, after it started to green up.  I told him it was not labeled. We discussed it. Then he decided to go ahead and do it.  It burned the alfalfa tops, but then the alfalfa grew out of it.  I suspect it affected 1st cutting yield a bit, but it was better than a weedy alfalfa stand.  I think the burning was less than we had feared. 

You might find the same thing here.  It is not recommended as an abundance of precaution. However, since it is done, the damage may be less than feared. 

Again, keep us posted.