I have been a pagan for quite a while, trained and initiated Gardnerian, which is, I think, one of the trads that have a tendency towards being more socially conservative. I was never personally aware of anyone within Gardnerianism who was poly, but in the greater pagan community there was at least one publicly open triad, (not sure, but I think it was a mythical unicorn!!) and several paired couples with open marriages. It was much more accepted there than in the general public. Most who do not participate feel that their reasons are their own, just as their religious beliefs are their own; they feel it's not for them, but don't think of it as 'wrong' or morally at issue. I also agree that there is a community of like-minded individuals that intersect and overlap (pagans,geeks, fans, historical recreation, etc.) that tend to be extremely open minded and question many of our ingrained ideas. They seem to have a higher percentage of people who are involved in a poly lifestyle, are open to going there, or at least do not have a problem with other people who do.
I live in a fairly small midwestern citie (US) and many people know that M (my poly bf) lives with me and D (his non-poly gf). A couple of our neighbors know, and most of our friends do, too. We haven't had any negative comments, at least not to our faces. Just "I've got no problem with that, whatever suits you. It's not for me, but that's ok."
I wish you well!!