This would simply pull the minoxidil(along with whatever else was sitting on the surface of your scalp) into your scalp blood vessels and then into systemic circulation. This would not be good. You might as well take oral minoxidil. The whole point is for minoxidil to dissolve slowly into the scalp so it exerts its effects on the follicles for an extended period of time. DMSO would radically reduce that effect and send everything straight to systemic circulation.
If you dont believe me, buy some DMSO and stick your finger tip in the bottle. In about 15seconds you will taste it in your mouth!