I'm not sure if it is optimal to "build it in", Ben, since there is no single optimal way to use it, or any singular set of lighting conditions. It would be tantamount to having a pedalboard come with a built-in wah, fixed in one location on the board. A "convenience" of a sort, I suppose, but not necessarily ideally-suited to everyone's needs.
Having said that, a moulded cable that branches off to a signal cable with a phone plug at each end, and a control cable with a nicely encased photocell of suitable resistance range and sensitivity at one end and phone plug at the other end to plug into expression-pedal jacks, would be a nice thing to be able to pop into the store and buy. Put some velcro on the back of the photocell module, and a piece on the pickguard, and away you go.
One of the nice things about photocells on board is that they let you do something different with your foot if you want, as well as providing a different feel.