I'm already aware of the interaction between Mage Armor and features that grant an alternative AC calculation such as a Monk or Barbarian's Unarmored Defense, or simply wearing Armor. However I haven't been able to find any definitive answer on how it interacts with a Tortle's Natural Armor or with any other creatures whose Base AC is also derived from a flat Natural Armor value in the same way.
Relevant rules excerpts:
Tortle (MPMM, p34)
Natural Armor. Your shell provides you a base AC of 17 (your Dexterity modifier doesn't affect this number). You can't wear light, medium, or heavy armor, but if you are using a shield, you can apply the shield's bonus as normal.
Mage Armor (PHB, p256)
You touch a willing creature who isn't wearing armor, and a protective magical force surrounds it until the spell ends. The target's base AC becomes 13 + its Dexterity modifier. The spell ends if the target dons armor or if you dismiss the spell as an action.
Emphasis mine in both cases. Since both of these effects specify an override to the typical Base AC calculation of 10+DEX it's unclear to me which effect takes precedence. Does the term "Base AC" even have any special meaning? I'm curious as to whether there's any rules text that covers this scenario. RAW-only answers please, I'm not looking for DM rulings but for some actual material I can refer to or a definitive statement that such rules text does not currently exist.
For the curious, a player jokingly mused on whether you could charm a Monster with low DEX but fantastic Natural Armor, make it a willing target of Mage Armor in doing so and thus cripple its AC by overriding it and I actually have no idea whether this works RAW. In terms of my personal DM ruling, absolutely not, but the RAW justification revolving around the term "Base AC" struck me as an interesting subject.