Well that's not so bad.... and you are going to be gone for a week? You could almost set up something like a terrarium. If you create a micro climate where the humidity can stay relatively high, water them well, and have humidity trays for them, it would likely be fine. So figure out how much space they will take, get some pvc pipe and make a frame, get some clear plastic you can cover the frame with, and make a tent of sorts. Make the frame just a little taller than your tallest tree, so there is less air volume in there.
The plants won't draw as much water if they are not expiring it into the air. The more humid it is, the less they will draw - and the less the soil will evaporate it as well. So the key would be to make it as contained as possible, don't just drape the plastic over it, but make it as sealed as possible.
Don't put any plants with any illness or infestations into that space though, because it will become a breeding ground if you do and it will spread to the other plants. So check them over carefully for scale etc before putting them in.