An official timer would be great and a logical solution, but I don’t think ANet will ever do it. Removing the ability to get a chest while guested might work, but I don’t think it would solve the overflow issue at all – there are a lot of servers that have enough people to hit overflow just with the regular server population.
Having all events go at once, even overflows, actually sounds good. That way, even if you get stuck in an overflow you don’t miss it. This would pull people into the chest zones all at the same time though, so the other zones would probably be emptier for however long it took to finish the meta event.
And making the events more frequent would also help. EX:
Shadow Behemoth events are at least an hour apart. Everyone has to time their play in order to get it. People camp, the map goes into overflow, it’s a bad scene.
Frozen Maw happens very quickly (every 30 min? ). There’s almost never enough people to send the map into overflow.
If all the events spawned more often, there’d be less pressure to be there for an event, and fewer people would camp them. Problem solved.