And even if you were interested in developing romantic feelings, it isn't something you can "build up" on purspose, from the scratch.
As for us, we're open to embrance romantic feeling should they bring up with people we knew swinging, but we're not activelly looking for them. This is much like trying to meet someone you don't know out wanting to get a new friend... no one make friends this way. You meet people, and luckily, very few of the people you get to know become friends.
As for today, should someone bring up telling us he/she/they want a romantic reationship with us, even before having fun togheter... we would run away, as fast as we can!
Again, this has nothing to do with being swingers or keen to poly relationsips. Before marrying, when I meet a gal, I was looking just for fun, if she were leaning on me with her expectations for a relationship, I'd have run. And when I meet my wife, we was up to have fun for a weekend that turned into the longest weekend ever. When we engage in a poly relationship, we had this girl friend, swinger, we enjoyed hanging up togheter and at some point we realized it was way more than just having fun.
So, I have no clue on how to "make this happen" or "make this work". I think you just cannot devise an plan beforehand such a relationship. At most, to be up to let it happen... when and if it happens.