We have read about them on the websites of some relocation agents, but we have never had anyone on this board who actually successfully got one!
According to the relocation agents you need to be a heterosexual couple who have been (and can provide strong evidence of) cohabiting for some time, and who are prepared to sign a written undertaking that you have plans in place to get married. And even then you your partner won't be able to do anything (work (even voluntary work), study, etc) while in HK and they will need more paperwork to exit and re-enter.
However, "Prolonged Visitor Visas" are not mentioned on the (otherwise comprehensive) Immigration Department website (http://www.immd.gov.hk). And, as I said, no-one here has ever reported actually getting one (and this question does come up very regularly).