Are you talking point to point wireless between two nodes or are you talking about providing wifi to the concert audience? Cause one is easy, and the other will require an elaborate mesh wifi network for it to even REMOTELY work.
Since you mentioned LRE's, I think you just want point to point. I would just get some direction parabolic antenna's and point them at each other with WIFI bridges on the back side. These set-up's can shoot for miles and in a concert all you need is a line of sight.
Here's one i've used before:
27" Parabolic 2.4-2.5ghz Antenna 23db gain If you are only going to have a couple hundred feet between antenna's then you should probably go with some cheaper. I've also used this one before with good luck:
Small 2.4-2.5ghz Antenna 15db gain In my limited experience with WIFI, the quality of the connection has always boiled down to the antenna and radio amplifier used. For the most part they have the radio amplifiers perfected but the sheer mass of crap antenna's floating around still boggles me.
As far as it being mobile that's easy. Take a 5-gallon bucket, a 2-foot piece of PVC, seal the bottom of the PVC with silicone to prevent leaks, then fill the bucket with concrete. You can drill the piece of PVC sticking out the top so you an put a set-pin in to keep the mast in place. If it needs to be sturdier you can always use sandbags or bolt the mast to some railing too. I'm sure it'd sway in the wind some but it would probably fit the bill for what you need.
Another method that is more sturdier but more of a PITA to move is using a 4'x4' piece of 3/4" plywood with a pipe flange bolted in the center and about 1-foot piece of pipe screwed on it. You then put eye-hooks in the 4 corners and use tension wires (with turn-buckles) that hook up to the mast about 2/3 of the way up. To weight it down against wind loading you can use cinder blocks or sand/water bags. The water bags are a nice idea cause you don't have to haul them up the stairs, just a long hose.
