North of the Rio Grande, I see the best opportunity for this is with Cherokee in a disUnited States scenario. After a Georgia/North Carolina conflict, the Cherokee become a buffer between the two states, heavily favoring an alliance with one or the other state.
The wealthier Cherokee begin to marry into the elite of the favored state, establishing their culture to a larger degree in the state. Instead of a unique Cherokee alphabet, *Sequoyah creates a standardized spelling with the Western alphabet. Cherokee literature develops and becomes popular, and becomes a point of 'national pride' in the favored state.
After repeated epidemics/military incursions from the unfavored state, the Cherokee move in larger numbers into the allied state, integrating into the culture there. As their nation disintegrates, the elites of the favored state begin to push the language of their Cherokee brethren in school as a point of nationalism, eventually making it a national language.
The Cherokee culture is less preserved, but ironically the language becomes much more widespread.