Right now, it is my understanding that guesting – when it is implemented – will be restricted to servers where you currently have friends.
If guesting is, instead, made without this restriction, it would potentially alleviate some of the issues with dead events or zones. At worst, those wanting to could seek out realms where the zone they want to play is more active – at best, larger scale zone efforts could be coordinated through the forums.
I know there are downsides and roadblocks to this – and that guesting has to actually be in the game first – but I think it is a compromise worth considering.
The other option, which has been suggested already, would be use of the overflow mechanic – only in reverse – to ensure zones are populated.
Either way, I think it is time to do something. The 5 zones listed as “high” in North America are struggling pretty bad when it comes to dynamic event participation right now.