I wonder how that would work when only a few years earlier Peninsulares were kicked out after trying to overthrow the government and return to Spanish rule. At the time, wouldn't Mexico be under centralist government? Would they accept so many liberals? I wonder if this means that Mexico becomes a federal republic earlier, it could certainly have massive changes. Also someone else mentioned Cuba, Mexico would probably want in on the pie if Cuba tries independence with the sudden influx of liberals. At the very least sending volunteers lead by these exiled liberals.Realistically, Carlist Spain would not even try lest they face the might of the USA. Besides, an alternate idea might be for the Spanish liberals to flee to Mexico with US assistance. I imagine that they could become prominent political voices and influence Mexico to be more liberal early on, especially contrasting the Carlist Spanish's ambitions or at least talks of reconquest.
I also wonder if this would spell certain doom for the Carlist government too due to the further crumbling of the Spanish Empire.