Interesting thought.
I would think that with the whole "penis envy" thing that psychologists/psychiatrists/behaviorists talk about, it might be a natural fantasy for some women.
But I think women are much less likely to view porn (unlike men!) and hence more unlikely to encounter shemale porn or photos that might stimulate their fantasies.
Personally, I think a woman with a tgirl is very hot!
Just a thought....