"Do you know what kind of response people have had on 'traditional' dating sites like pof when they advertise their 'polyness?'"
I'm not experienced with POF. OKCupid seems to be a mixed package; some people have more success on it than others. It is remarkably poly friendly, for being a mainstream dating site. As for PolyMatchMaker, there's obviously no issue there as it's assumed you're poly.
Hopefully you will have a successful experience as you start off on this new journey. There are many positive signs out there.