Nowhere in Latin America is alternative rock... with the exception of a few crossover cuts... particularly popular.
Further, where rock in general is popular in Latin America (that is, one or more stations playing some kind of rock), the shares are moderate to low. However, many of those stations do well because the listeners are in the highest income groups* which advertisers seek. And those listeners who have good incomes do not migrate to the US; immigrants in their vast majority are partisans of their country's equivalent of country... for example, "regional Mexican" banda and norteña music in the case of those from México.
So trying to get Hispanics to listen to a kind of music that is very remote from their social, cultural and family culture is a rather useless task unless the whole term "Alt" is redefined.
* As an example, in Mexico they separate ratings by A, B, C, D, and E socioeconomic classes. A, B and C+ are the targets that most advertisers there seek. In Mexico City, a "regional Mexican" station with a total share over 15 bills less than an AC station that plays English language AC songs and has a 3 share or so.