Being able to park at RIV is dependent on what "Dining at Riviera" means. If it means you have an ADR at Topolino's, then yes you can park there. The firm rules say something about an hour or two before and after your ADR, but I don't think they enforce the after the reservation time. As far as before, if you showed at 8AM for a 6PM reservation, YMMV. If you are planning to dine at the bar, Primo Piatto, or get snacks at La Petit Cafe, I doubt they will let you park there. The rule is you need an ADR. The monorail, skyliner, and Epcot resorts are pretty strict on this. You need to really get lucky at the guard shack to be let in without a room or ADR at those resorts.