...I've really noticed that those who report serious loss of erectile function whilst on hormones, tend to be those that hated using their birth genitals pre-HRT. Whilst, those who enjoy or don't mind using their genitals pre-HRT tend to retain function (if slightly or moderately impaired). So - mind over matter, ultimately?
Perhaps that's another reason why some trans porn stars retain function, because they enjoy their work or at the least are not offended by their own genitals?
It seems that with men and women, arousal and attraction seem to be the key to getting turned-on, and this "software" in the head powers the *ahem*...hardware in the engine room. The less issue the software has with hardware, the better the hardware works perhaps?
Hannah from Sweden hosts a vlog called "Ask a Transexual" and she seems to provide some interesting and in-depth answers to questions such as this.
She's also an adult movie actress and is bisexual, so she seems (from what she says) to need to carefully manager her hormone regime to these ends.
http://www.youtube.com/watch?v=J8z1zH2mphk&list=PLCE9D708733D50B8FAnother such actress - Bailey Jay - also has commented that she had to experiment alot with hormones when she began HRT in order to retain function, and also to regulate her fluctuating weight gain, without negating the feminising effects. Once again - very careful and specific management tailored to her specific needs.