That's oddly specific, haha.
Based on what I know, the kind of person you are looking for would most likely be IEE or maybe EII-Ne.
I don't know of any ILI who is into polyamory. I know of a guy who is and seems to be LIE Sx/So, but he is definitely the Dom.
Hence I find Gamma NT is rather unlikely going to be the type who would fit this profile.
Most people who are into Polyamory and/or "open marriage" seem to be IEE Sx/So 7.
In general, it looks like polyamorous circles are full of contraflow Delta NFs.
Could have something to do with polyamory being, in an idealistic way, kind of like hippy love – "free love".
valuing (and
Demonstrative) individuals (esp. the males) are not as common in those circles, because they typically are rather possessive of their partners and can be jealous more easily.
So, Delta NF it is (most likely).