Dimethyl Sulfoxide (DMSO) is an organic solvent (I may not be a vet but I am a biochemist) which means that it dissolves a wide array of molecules that are normally in a stable form. DMSO is therefore capable of carrying toxins with it into the bloodstream if there is a point of entry. The bright side is that DMSO itself is not overly harmful as is a potent antioxidant and anti inflammatory agent, hence its use for reducing swelling. Although it is approved for use in dogs and horses, the FDA has refused to approve its use as a topical application in humans because of its solvent properties (so use gloves too).
So whatever you mix it with will end up in the bloodstream if it comes into contact with an open wound, most linaments/topical applications are great on the surface of the skin but not designed for the circulatory system.
I would ask a vet about using DMSO on open skin. I suspect most will say no. As Starline said, her vet recommended this although she didn't mention whether this was for treatment of an injury where there is broken skin or not - just be careful with DMSO.
Good luck, I have seen many horses have good recoveries from bowed tendons, just be patient recovery time will vary depending on the extent of tendon damage, again, a vet will give you the best idea on this timeframe for your individual horse. Keep us updated!
All horses deserve, at least once in their lives, to be loved by a little girl.