The UN Security Council is somewhat similar in that respect, though comprised of more members. New matters are approved by majority vote, but the five permanent members can veto anything that gets approved.

My proposal is somewhat tongue in cheek, hence the names from the Roman Republic - but I do think the idea is sound.

An alternate, simpler implementation would be to simply require that all changes be unanimous. I like the idea of a Tribune of the Plebs though because it gives a second means of a voice should a sizable minority of members of the sub-communities strongly disagree with the direction of their representatives.

and in case it wasnt clear i was referencing the US tripartite governance model

The current governance model is mostly around consensus on the dev list. There's a "technical board" elected every release cycle from among the committers, to act as a tie-breaker when needed, and with veto power over DEPs (Django's equivalent of PEPs) and adding new committers. I don't know of a case where that veto power has ever been exercised, FWIW.