Odd - would this not result in a male resonance and one sounds like a high pitched guy because pitch is after all not all there is to a feminine voice. I guess it makes sense to first not strain things too much but after a while, one will have to do something, not? I mean - there are some people who did have voice surgeries that I have heard (not from yeson but from others with a similar technique) who do not really sound that female despite the higher pitch, because they did not change the rest of their voices.
If it would be so easy - to just speak without any effort and sound female, it would be a quick fix surgery for the voice, but that seems to me to be unlikely.