Interesting replies! I'd like to quickly add some of my opinions to discuss it later:
1 - Most of the revolts in Brazil by that time, like the other big countries in America, e.g. Argentina, Mexico, USA, were not about Conservatism or Liberalism (I think that would be better suited to discuss European situation), but Political Centralism against more Regional Liberty ("Federalsm"). Issues about political Conservatism or Liberalism woud only be collateral dammage, in my opinion.
2 - As someone said before, Uruguay was far from being an uncontested Spanish/Argentinean territory. Portuguese Colônia do Sacramento was founded in the early 17th century and it changed hands from Portugal to Spain countless times ever since. It was far from being a foreign land to the Brazilian/Portuguese.
3 - As for the ethnical/linguistic problem, I don't think that would be a big issue either. The farmers from Rio Grande do Sul would most probably migrate searching more fertile lands in a Texas-like moviment. I'm sure that the Spanish "Platino" culture would not be totally supplanted, but, as I see, Brazilian Cisplatina would "just" be another unhappy Brazilian province.