This is a subject not often talked about, beyond the fetish community, not sure if the best methods have been shared. I have seen several sites where experts referred to it as "Viagra for hair".

I would recommend for anyone who likes PIV sex to use condoms WITHOUT spermicide (they are readily available) to accumulate the semen without killing the beneficial sperm. Also sperm can live up to 5 days if maintained near body temperature, so you can accumulate multiple times over a few days if you have a 100F storage place and then apply them all at once for covering long thick hair.

When you consider the pleasure aspect contributing to hair improvement, it certainly seems like a win-win for both the man and woman.