Answer: Iroquois Governance Structure's Impact on the U.S. Constitution.

The book Lies My Teacher Told Me by James W. Loewen has chapters on Native Americans that speak to this question. I can't remember if this was specific to Iroquois peoples, but a native governance structure is shown in this book to be a model for how the colonies began to have better relations among one another.

Here are some more references:

Title: The Iroquois Confederation Constitution: an analysis.
Source: Publius [0048-5950] Lutz, Donald S. yr:1998 vol:28 iss:2 pg:99

Abstract:

The Iroquois Confederation was not an influence on the U.S. Constitution, but it is worthy of study as an independently developed political system with the oldest surviving constitution in North America. A systematic institutional analysis of the Great Binding Law, the orally transmitted constitution of the Confederation, reveals, among other things: tribal inequality despite their formal equality under a unanimity rule; a high level of responsiveness despite a nondemocratic, elitist method for selecting leaders; many ancillary institutions for achieving a traditional form of consensus rather than simple majority rule; two means of elevating men to the Confederation Council, each a paradoxical blend of the pre political and the post-traditional; the first use of a formal amendment process in constitutional history; and an underlying "code of imperialism" that, together with the second method of selecting Confederation Council members, transformed a defensive alliance into a potent actor in North American history. Overall, the Confederation institutionally approximated an Aristotelian "mixed regime" which, despite its creation under circumstances the Iroquois describe in Hobbesian terms, was quite libertarian.
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Title: Debating the origins of democracy: overview of an annotated bibliography.
Source: American Indian culture and research journal [0161-6463] Johansen, Bruce E. yr:1996 vol:20 iss:2 pg:155

Abstract:

A bibliography documents the debates on the influence of Iroquois and other indigenous confederacies on the ideas of democracy in the United States. Critics view multiculturalism and political correctness as central to the discussion. Some consider American culture as a derivative of European culture, while others maintain that Indian democracies influenced the political theories of the founding fathers. R. David Edmunds discusses Native American influence on the US constitution in 'Native Americans, New Voices: American Indian History, 1895-1995.'
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Title: Forgotten founders.
Source: Omni [0149-8711] Bosveld, Jane yr:1992 vol:14 iss:5 pg:33

Abstract:

Anthropologists and researchers believe that the constitution of the Iroquois Indians influenced the making of the US Constitution. The Great Law of Peace guaranteed equal representation for each of the five tribes, political and religious expression, and other democratic ideals.
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Title: The Iroquois League, the articles of confederation, and the constitution.
Source: The William and Mary quarterly [0043-5597] Payne, Samuel B., Jr. yr:1996 vol:53 iss:3 pg:605

Abstract:

Donald A. Grinde, Jr. and Bruce E. Johansen maintain that the Iroquois League influenced the formation of the American union and constitution. However, a close scrutiny of their thesis shows that it is untenable. Anglo-American leaders knew too little about Indian political practices to be influenced by them. It is doubtful whether greater knowledge would have made a difference, as Iroquois society was vastly dissimilar to Anglo-American society. Moreover, the American constitution is a federal union instead of the Iroquois confederation system of government.
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Lastly, regarding the book Lies My Teacher Told Me, to reiterate a bit, the Native American influence that Loewen writes about is on a form of government that precedes the constitution. So, when reading very specific arguments that 'founding fathers' like Jefferson never had meetings with Iroquois leaders on such-and-such a date, they're ignoring other scholarship and the role of earlier documents and social practice in forming the constitution.