Hi all. The big problem with a subtle approach is that if she reacts badly, it's more awkward to later say how important it is to you. And, she might not like the idea for play, but could come to some accommodation if she knows it's important to you.
As Dan and Jenn said, you really should bring this up before you get serious, or you're in for a WAY rocky road later.
Lastly, do some research and think about what your desires are. Some people xdress as a fetish only (inside, bedroom). Some "underdress&quo t; more regularly. Some are happy being men but occasionally completely crossdress in society (either at conferences/local groups, or plain out in the open). And, of course, some decide they're transsexual and go for body changes. Having some idea (at least an estimate) where you are will help you both determine what you can deal with (or -- better -- what you can enjoy about each other!).
See www.espritconf.com and www.sccatl.org for example conventions where you can lea