That's going to be really tough.
Do you have a small set of common (flat) screwdrivers? I would try using one to unscrew. If that didn't work, I'd try wrapping a little piece of plumbing tape around the hex key to try and 'fill' the head of the screw to get a little grip.
Also: try using the other end of your hex key; I spent a while trying to get some saddle screws out of a strat once, to no avail, only to then drop the key, and try the other end, and then all of them came out without any trouble at all. The allen key was malformed, not the hex screw.