Depending on the angle of the misting nozzles you use, I don't think overspray would be that much of a concern on a build of that size. (i'm assuming most of it will be directed towards the center?).
I'd be more concerned with the tank's surface area, misting, and fogger acting as a humidifier for the whole room. might not be an issue, I don't know.
One other thing you could do, is instead of thinking of it as an open-air palludarium, maybe think of it as a closed on two sides (the sides along the walls), you could either protect the walls with something, (maybe those big 4x8(i think?) plastic panels they sell in Lowes/Home Depot for putting in shower walls and such
Also, you may not need to worry about the humidity as much if you can use the mister a couple times a day, just to keep the plants from drying out (but then this runs the risk or rotting the ash branches).
If nothing else, you could always look for plants that do well at a lower humidity level.
Apologies if this is coming across as too critical, that's not how I mean it at all, I'm just trying to throw ideas out. I think this is a pretty cool build you have planned, and want to see how it works out.