A very risky thing to do. I've seen a few guys get positive results with it but from what I've read the results go away once you stop using the product. You can try looking at this thread for more experiences and results:
http://jefffsbeardboard.yuku.com/topic/23391/Minoxidil-Summary-and-Experiences#.WLvAVW8rLcdThere is also a blog my a transman named Oliwer where he documented his minoxidil use here:
http://loadingbeard.tumblr.com/ with his case you can definitely see that he already had very good coverage and the minoxidil was used to bolster what he already had. You can find his full transition blog here:
http://loadingoliver.tumblr.com/ Perhaps his usage of the product helped and perhaps not. 13 or so months on continued testosterone usage will cause the beard to mature more. Some men don't fully develop their beards until their 50's.
From what I've seen with transmen and my own experience you can expect to see some massive results around the 1 - 3 year marker. My beard still had a ways to go but it's at a stage where I could say it's "full" albeit thin. If you'd like I have a link to my beard documentation in my signature.
Above all else. If you DO choose to use this product please be careful. You never know what side effects it can cause. Happy bearding!