I tested it out real quick with a custom nation: A Native Council with -20% Land Maintenance NI and a Grand Captain, raising War Taxes. Theoretically being Mayan should've allowed me to get it down to -110% through reforms, but there's still a UI conflict where being a Native Council and Mayan/Inti at the same time doesn't let you access both screens.
In any case, I don't think it would've mattered. Even at -100% Land Maintenance I still paid money for my units. (albeit almost nothing) I think the base cost of regiments enforces a minimum ducat cost you still have to pay no matter how low the maintenance modifier is.
However, by taking Quantity Ideas and a -20% Regiment Cost NI I was able to squeeze that number down to .01 ducat per month per infantry. I'm not sure if that's the floor or if you can go even lower.
So stacking both Land Maintenance AND Regiment Cost reduction may be successful. In theory, the lowest-cost build possible is probably a Tengri Native Council with -20% Regiment Cost and Land Maintenance NIs, -20% Infantry, Cavalry, and Artillery cost NIs, with a Grand Captain hired and War Taxes raised, trading in Iron, Livestock, and Salt, as well as Quantity, Economic, Expansion, Administrative, and Defensive ideas.
All that put together between policies, ideas, etc. would give
-65% Regiment Cost, -20% Infantry Cost, -30% Cavalry Cost, -20% Artillery Cost, and
-150% Land Maintenance modifiers. If you play on Very Easy you can bring that Regiment Cost down to -98%... But who wants to do that?
In theory, if you could have a parliament and a native council at the same time, or be Revolutionary and have a Parliament, or somehow have Estates as a Native Council or a Revolutionary Government, or all three of them together, you could stack things even higher, too. You could also enjoy the novelty of how a Revolutionary Tribal Parliament of the Estates-of-the-Realm is somehow the most efficient way to supply an army.
