Why not meet halfway and discuss openly about your polyamorousness. Then the question or hinting won't be so daunting. You might learn out of the box they aren't interested.
For the record I am not saying this as a deception, simply meaning, be open about your overall position when engaging with people and you won't surprise them with something as potentially shocking as an open relationship.
I agree with you, letting things progress naturally, when they don't even know there is a possibility.. Chances are lower.