Optoelectronics + Moogerfooger = sounds never heard before
http://optoelectronics.perkinelmer.com/content/RelatedLinks/DTS_sensorportfolio4-Page.pdf
Not completely unheard of in the controller realm. The Morley pedals all used a photoresistor instead of potentiometers and then there is the Dbeam on some modern synths.
Even photocell theremins are available.
Like ribbon controllers, touchpads and capacitance sensors, optical sensors haven’t been very fully realized.
One of the cool things about using photosensors are they are isolated devices and when used as switches don’t make any noise when switched. (Provided the circuit is even moderately well designed.)
The slalom pedal on my Farfisa is a photoresistor. I agree about photoresistors being under utilized. Low cost, low maintenence…What more can you ask for? 
They can be easily made for much less than that.
Photocell - <$1.00
Aluminum box - You can probably find this in places like Radio shack for a few $s
female plug receptor - a few $s or less.
Wire and solder <$1
Total: Less than $5
What is more interesting are the techniques of using these. Consider multiple photo cells in an array. All sorts of sequencing options here.
What might also be nice is one of these that can be placed on a keyboard or two with book light type lights and two photocells perpendicular to one another.
There are also vactrols which have an led and photocell in one unit. These cost about $6 and can be made into photo envelopes and envelope followers.
Vactrols create a unique envelope and are used in many synth modules.