There's an easy way to get around this. Or rather, these types of zone access restrictions can be grouped into two categories: Long, predictable cycle, and short, random cycle.
Long cycle events (things that can take anywhere from half an hour to a year earth time) must be predictable, or they are nothing more than a nuisance. The first things that pop to mind are a strange crystalline substance that is liquid in it's natural state, but when exposed to enough moonlight (>75% Waxing && >75% Waning) hardens into a clear, crystal bridge. Another possibility (given free reign of our imagination, since this level of zone craftsmanship would be prohibitively expensive to make) would be seasons that stretch for months real time. During the summer, a certain lake dries up, exposing a small cave, at the base of which lays the slumbering Ruby Weapon, tasked with guarding the ancient technologies hidden within. Killing Ruby Weapon would be a server-wide en devour.
While very different in scope, these two examples share something fundamental. They are regular and predictable. Anyone who knows the pattern can plan ahead to accomplish things at the right time. Similarly, the scale of the content is proportional to the duration of the wait. If you are asking the server to wait days to accomplish something, the reward (tangible or emotional) has to be worth the wait).
On the other hand, short cycle events (those that last for minutes at a time) are better off left to promote choice. Do you wait for the impediment to pass, or take a longer, potentially more dangerous route. For myself, I personally think these types of dynamic content are better suited for eye candy. Keep dungeons and terrain interesting, but momentary impediments are just going to be that, impediments. For myself, I would much rather have static, but interesting, environments than constantly changing, but unpredictable environments. The best example I can think of is Pso'Xja, which had some of the most bizarre and wonderful dungeons I've ever seen (walk-through walls, invisible holes in the ground, vertical dungeon that you constantly have to go up and down holes and ladders to get around in).