From my understanding of the research into cultural anthropology, flat hierarchies are often enforced through strict social norms including what we might call today societal shaming or verbal abuse. That being said, there's a reason those pre-agricultural cultures enforce flat hiearchies (to prevent jealousy and murder, chiefly).
It would seem the solution in polyamory is a thorough discussion of boundaries.
However, I doubt that it is ever possible for any relationship that starts as monogamous and grows to be poly to be non-hierarchical. The wife will always be your primary partner, and there's nothing wrong with that approach.
Thanks for sharing your insights.