WIY: Maybe show them the Wikipedia article on genuflection then ask if they still think it’s not a problem? You don’t need to find a Rov who knows of the Goyish concept. If you tell a Rov about the concept (and he believes you) then he should be able to decide whether it falls under Chukas Akum or no.
Writersoul: Getting on one knee as a means of supplication (and you are asking someone to marry you) without a doubt in my mind constitutes an Assur form of genuflection.
Wolf: I think I once made that same claim (that a proposal isn’t Kiddushin because it’s D’varim Sheb’leiv Uv’leiv Kol Adam). In fact, I think I made it in the CR. I’ll have to think about this more. It could be that you are right.