How would one replace the 10k pot with photo resistors and an LED? Two photoresistors and two LEDs operating out of phase from each other? and how about approximating that 10K value? (higher vaiue pots will work, but there is a large middle range of the pot where there is no audible response from the circuit.)
I'm also wondering if another circuit will be closer to what I'm looking for.
(http://www.jiggawoo.eclipse.co.uk/guitarhq/Circuitsnippets/gargletron.gif)
Looks to me like you need to check out envelope detectors, Blackie. There is a LOT of info out there, google & youtube.
There are also some circuits similar to this that are optically-controlled already, but if you have this built and want to mess around, have at it. I believe it would take 2 LDRs and some experimenting, and as you said, out of phase.
You'd probably want to trim out where the pot is to get 'into the range', and then set up your LED/LDRs to operate inside the limited 'sweep' you have left, by biasing the LEDs and so on. Keep looking, I KNOW there are things that do this already to check out, just nothing comes right to mind! It's a good idea.