Hmmm. Is there some way to mention your marriage in a casual way? "My wife was asking about you the other day ... nothing bad, we've had an open marriage for awhile." That might suffice for one conversation. At some future conversation you might get a chance to bring it up again. "Hey, you know how I said my wife and I have an open marriage. I was wondering how you felt about that sort of thing?" That might suffice for that second conversation. Maybe in some third conversation you'll have opportunity to ask the big question. "You know, I've actually been wanting to ask you out for some time ... Is that something you might want to do?"
I won't pretend to be some kind of poly dating wizard who knows how to ease someone into the subject of poly and open relationships. You just approach the subject as best as you can, a little at a time. Give Estrella time to digest each bit of information before bringing up more. Who knows how it'll happen in real life. Maybe all "three" of those conversations will end up happening in just one conversation. There's really no magic answer though; it certainly makes sense to proceed cautiously, whereas if caution stops you dead in your tracks, then you'll never know whether Estrella might have been interested in spite of the unusual arrangement.
Worst case scenario, she says no. That doesn't mean she doesn't like you and I wouldn't take it that way. Just be cool about it and say, "That's okay, I just had to ask." And then continue being friendly with her if that's okay with her. A friendship is quite valuable even if it never goes anywhere romantic. Just respect her right to think for herself and make her own decisions. She's probably perfectly capable of being objective about things, she seems like a sensible type. "No" could mean nothing more than, "I just don't happen to be romantically interested." That can happen in monogamous dating as well as polyamorous dating.
Maybe someday someone will write a handbook about how to go about these things. Right now, though, poly and open relationships aren't widely enough accepted to have a lot of published/professional support. We're kind of forced to figure things out as we go along. If we're lucky we get experienced advice from folks who've done a lot of poly dating. Alas, however, I'm not one of those people. Shoot I barely have any dating experience at all, let alone poly dating experience. I just know that confessing my feelings for someone has served me well in the past. At the worst it's at least drawn out a thoughtful response from the other person.
Love means never having to say, "Put down that meat cleaver!"