Isn't there a clause for Grand Servants that Divinities don't qualify? And Romulus was already "the exception," as stated in his bond:
I mean yeah, but you also have to take into account his unofficial MO:
X is impossible to do because Y. Character Z does X regardless.
Everything is an exception until it isn't. Scáthach could only alter her Saint Graph to get her summer outfit via Primordial Runes? Bam! Santa Alter Seiba.
Foreigners HAVE to be Outer Gods? Bam! MHXX.
Mystery gets weaker with age? Bam! Don Quixote/Tesla/Edison.
Grand Servants need to meet a specific criteria to qualify:
From the Nasu/Takeuchi
Q&A: "Reaching the top of their respective Class, by meeting high standards in both power and legends. There are more than one candidate Servant for each Class, they're ultimately chosen according to each era's
target to be overcome"
From Romulus-Quirinus'
translated Servant Profile: "Normally, it is unthinkable that a Divine Spirit would be summoned as a Grand Class servant."
The wording
technically doesn't say impossible, But here's the thing: Cipactli would be more akin to a Divine Beast than Divine Spirit, and Tetzcatlipoca
technically lost his Authority as a Sun God, whereas Coyolxāuhqui was
fragmented and thus in Nasu-land doesn't have the full Authority as a Goddess (see: Ana/Medusa/Gorgon), so they technically wouldn't be full-fledged Divinities, but rather on the scale of demi-gods like Heracles (who in theory could also manifest as a Divine Spirit).
Plus, the Mesoamerican gods are an exception to some of the chains binding Natural Spirits such as Divine Spirits ordinarily: they're Alien existences from Gaia not unlike Altera or the Machine Gods.
Except that unlike Chaos or Velber who actively invaded Gaia, they just so happened to crash there, and with the event accidentally decimating the Dinosaurs who previously held the
Seat of Primacy, they are less rejected by Gaia for those two reasons, eventually becoming subsumed like the Olympian Gods but still maintaining that Foreigner qualifier.
Tl;dr: The Mesoamerican gods are exceptions amongst the exceptions for a lot of reasons, and the potential candidates would conceptually lack some of the characteristics that would define them as Divine Spirits; plus Lostbelt 7 is an oddity among the Lostbelts in that it lacks a Fantasy Tree. Plus, Nasu has a track record for breaking impossibilities so really, who's to say?