What you're looking for is something like this:
It is simply a socket of the appropriate size with a lever arm on it, designed to fit into a tight space like this. They're common if not universal in the stringed instrument world; Allen hex keys are most popular; most newer Peaveys actually use a wheel contraption that you can just stick any stiff pick or probe-like tool into and turn, no specialist equipment needed. These hex nuts were the thing to use before the Allen driver became popular, and many major manufacturers never switched over, so pretty much any musical retailer should be able to order you one of the right size without having to pay Peavey's markup.
The trick will be determining the proper size. If you happen to have a micrometer you can take your own measurement. Failing that, you might get friendly with a local M&P repair shop; they will have, and need, these wrenches in every size ever made, and can figure out which one you need by trial and error. If they won't sell you (or just let you keep) the one they found that worked, you at least know what size to special-order.
In my experience, which is limited, Peavey has some pretty good customer service. If you called and asked nicely, I wouldn't doubt they'd send you a small part like this just for the PR value. I have a Peavey bass amp from the 90s that was missing a knob when I got it, and they just sent me one in a padded envelope, covered shipping and everything.