I made some progress today. I used a router "box" template that I made from some 1/4" plywood and some scraps of 1/2" or 5/8" wood. First I made an inch thick block that was about 6" long and almost as wide as the neck pocket. I screwed it to the block that I laminated into the neck pocket.
The block created a relatively stable 6"x2" platform that I could clamp the box template to. (A box template like this works best with a router that has a circular bass. The outside edges of the box serve as the guide for the router. I have found these work well if what you need is a rectangular shape).
I did a little testing and was surprised how stable this was with just a couple of clamps to hold it into place. My router is pretty heavy and with 2HP has a ton of torque. The template was actually quite rigid however, and held nicely. Since the template was secured to a block which was screwed to the floor of the neck pocket the rout replicated the slope of the neck pocket exactly.
Some version of a this kind of box template may be very useful to others who need to cut a neck pocket or pickup cavity on a curved surface like an ES or LP top.
I put in one of the pickup rings to see how everything would line up... Starting to take shape ;-)