[Heaven's Night]

I don't know about topical application, but conventional HRT does cause shrinkage over time. That's probably your best bet and its almost guaranteed. Same with erections, HRT without practice to keep it functional will make eretions disappear as well. Note that both size and shape will change quite dramatically. After enough years on HRT you wont look "male" as much down there either and function is very different, so it sensitivity.

Pre/non op trans female genitals are not the same as what men have after enough time. Who would have thought? 

So there's your answer, conventional HRT aiming at female hormone balance.