So, basically, you want a fairly typical monogamous relationship, but to reserve your right to remain with your lover. Right?
Is there anything "wrong" with it? Heck, no.
Is it "possible"? Heck, yes.
It's certainly a positive factor that you're willing to approach this desire head-on. But I get the nagging feeling that broaching it upfront might be counterproductive.
If you weren't at all polyish, but were "looking for a possible husband," how would you go about that? I feel THAT would be the best approach: find someone worth dating, then date him (them?), & if he continues to show potential, THEN bring up your hopes.
I know, I know, this looks to directly contradict my usual spiel against "bait & switch" tactics, but I'm talking a few dinner dates & chat over cocktails, NOT getting him all primed up to move in/buy a ring/fornicate & then springing the GOTCHA at him.
See, my thought is that if you show the "I get to have another lover & you don't" card too early, you'll get agreement that isn't 100% realistic -- not that they're lying, but maybe that you are such an excellent attractive woman that a guy might be brushing it aside & it'd eventually become a problem when he actually needs to honor that commitment.
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My next thought is that
relationships change.
Let's say that you find the ideal guy, who totally loves you & has no problem whatever with your relationship structure.
Will she be part of his life? Will he see you with your lover in social situations or around your/his home? Will she have sex at your place &/or spend the occasional night?
And if it works out well & he sees how happy you are, how would you feel if he starts to feel that he's missing out on something important, & would maybe like to have that himself? What if he develops desire for your lover?
I mean, speaking just for myself, it reminds me of a family friend in the 1970s, who was married AND had a girlfriend (poorly kept "secret") AND had a string of affairs & last-call hookups. He got all kinds of righteous when he found out his wife would sometimes stop off at the VFW on the way home from work & have a couple of drinks with a friend... who was A GUY.

He called her fifty kinds of disgusting whore & ended the marriage, & seemed honestly hurt that he got so little sympathy.
While your proposed situation is hardly so extreme, the question remains: how will you "keep him in check" & hewing to rules that don't apply to you or to your less-committed partner?