I saw an outdoor rink with some people playing roller hockey in Parc Jarry last week (I dont think its that far from the Plateau). I'm not from Montreal either and I have no idea if its just pickup or a league. Maybe someone that knows more than me can chime in.
If you find anything tell me. I'm also a student staying in Montreal over the Summer looking for some roller hockey or softball action.
North side of Jarry park near St-Laurent and Jarry corner is the best place near that area for my money for some pickup hockey. The surface is painted cement I think although there are big cracks right now since they make an outdoor ice rink of it in the winter, but some guys usually manage to plug these cracks later on during springtime. Otherwise, the boards are really ******.