Personally, I can't see any reason why a hair transplant would come up in a conversation unless you'd had a procedure carried out by a hack.
I mean, would she tell you if she'd had, say, an eye tuck or botox?
In any case, if you did decide to tell her and she had an issue with it, you'd need to be asking yourself if she's the sort of women you'd want to go much further than a shag with anyway.