Is there a special procedure for mid-air refueling in the rain?
If you're VMC and simply have rain falling, the procedure doesn't change, and it isn't a big deal (talking fixed wing refueling with a boom here; other communities' procedures/capabilities may be different).  The receiver's windshield will have some amount of rain hitting it, but if it isn't particularly heavy rain then he should still be able to do the AR.

If the visibility is really bad because of the rain, or if you're getting into IMC, then that's the situation you have, irrespective of rain falling or not, and obviously you'd rather have better conditions for doing the refueling.  Going IMC momentarily while you're hooked up may not be a big deal, as you can still see the tanker, but if you lose sight then the first priority is to get positive separation between the two aircraft.