I think it partly depends on how "out" she is.
Personally if someone approached me it wouldn't worry me, but then I am in a semi-stealth mode. I don't advertise my situation, but I make very little effort to cover it up either, and so people can either know or not know as they choose... and believe me amazingly it does work! Some people do know and other clearly have chosen to ignore or simply not to notice. Either way they remain unaware and by and large things go on fine until some prize prat makes the mistake of making them aware of something they had missed. So I do try to make sure that whether or not people "know" of my past they are discrete.
If this woman is fully "out" - by which I don't just mean that it is public knowledge, but I mean if she is the sort who will "proclaim" her past then clearly you don't have a problem... It is a lot more tricky if she is like me, actually in semi stealth. In that case it would need sensitivity because while I wouldn't mind someone approaching me privately, I have got VERY cross when someone did it in a semi public way... for example by posting some trans specific comment on my facebook wall.
So you would need to do it privately and completely out of sight of any colleagues. Otherwise from my own reactions, I suspect you would be highly likely to get an official complaint made to your manager! (and I'm not joking)
In many respects it is probably best to accept that this isn't practical. The risk of embarrassment is too high.
Your other option of course would be to come fully out yourself and let her decide whether or not she wants to approach you.
None of these scenarios is ideal, and I do understand the frustration - however what you must remember is that those of us who have gone through this process do not often want to be poster children for some "movement" or "community". We just want to get on with our lives.
I come here because in the relative anonymity of the internet I can contribute something in the knowledge that if anyone from my real life stumbles upon me here and learns the shocking truth

they can only have done so because they came looking for it!

and therefore by implication they must have some interest

in which case I am cool with them knowing.