I know how you feel coming back from a trip hoping for blooms and getting blasted buds... I just got back from a 16 day trip to France and left all my 'chids in the care of my parents. Out of the half-dozen buds/spikes when I left, two remained when I got back. But, it's all part of the hoby, and so we carry on!
On the issue of running out of water, here's my solution (Well, not really my solution. I read it in a book on CP's a while back.): take a big plastic juice bottle, poke a hole in the side near the bottom, put some rocks in the bottle, fill it with water, put the cap on, and put it in your tank. When the water level gets below the hole, air can get in and therefor water can get out keeping the water level with the hole. While that might not make it through the long trip, it might see you through some shorter ones (I couldn't say for sure, not having tried this myself).
I think adding a lid, even if it's just a sheet of saran wrap suspended above the plants, will help a lot. That way you'll have way less evaporation, and, as I'm guessing was your intention, the mist'll condense on the top and trickle back down to the reservoir.
On a related topic, here's how I humidify my tank. My makeshift humidifier is very simple, consisting of:
-a piece of sponge
-a ziplock container
-a muffin fan
-two bits of wire to hold up the muffin fan
-water
The fan blows down onto the sponge, kept moist all the time by the water on the bottom. It works better than you might think. It keeps my 2'x2'x4' wide enclosure at 80+ humidity all the time. And I don't even have to run it 24/7. As far as quick, cheap solutions go, I'm quite happy with this one.
Hope this helps, and good luck with whatever you end up doing!

