IMO - the only time where it could be an issue with Riviera not having gondola's to board would be at park open. At park open, those heading to Epcot from Riviera would still have all the gondolas (empty) that start the day between CBR and Riviera. The efficiency of the system may make it a mute point. Even at the start of the day, you don't have a huge mass coming to the system all at once.
The only other two possible times of anybody having to wait any (short) amount of time for a gondola would be (a) at park close -- but that shouldn't effect those at the Riviera because the chances are those that exit at Riviera would far outnumber anybody that is trying to get a Gondola at that point in the night or (b) after any short closure due to weather -- but again this probably would be of little concern because most weather delays occur at low traffic times so it wouldn't take long for the Gondola to eliminate any crowds that waited.
Again, just my opinion, but seeing how they work in the areas I have, and the likely peak times for gondolas, the efficiency of them should handle just fine to avoid any issues at the Riviera without having to save empty ones (they could always do it in extreme circumstances, but why send empty ones if you don't need to right?).
Of course, just like a ski resort, there will be many empty gondolas even at peak, when a group is not paying attention and lets the gondola go by or gets in the wrong one (it always happens, hate being behind those people at a ski resort)