Bryan Shelton has some studies that show that IF topical finasteride is effective (there are some studies that failed to show that it was indeed effective), then it is effective, quite possibly, because it is systemically absorbed. Even if it's not effective, it might still be systemically absorbed.
But then that raises the question: if topical finasteride is systemically absorbed, what's the point of using it topically?
Lipoxidil.com sells topical finasteride and dutasteride formulations I believe. These formulations are supposedly delivered by way of liposomes, although some have expressed doubt that lipoxidil.com is really using liposomes as their delivery vehicle. Liposome formulations are very expensive. Dr. Lee was playing around with this for a while, and he was successful, but the stuff he came up with wasn't marketable. Why not? Too much money for the consumer. His liposome formulations were for minoxidil.
You can find these studies by doing a search for "topical finasteride." If Bryan is around, he can probably explain it a bit better.