Riviera, though, isn't a resort close to a park. Yes, you could get on the Skyliner from there, but that's not an extra a lot of "park crashers" apparently add.
Our experience has been that it's chancy arriving more than 15-20 minutes before ADR time for resorts within a short walk of a park, particularly when there are special events that day such as the Epcot festivals or MK holiday parties.