I wanted to try and create a "grass field" of LEDs that turn on when you run your hands through a bunch of fiber optic. A strand of fiber optic cable would be glued to each LED. I wanted to get ideas for sensing the presence of a user's touch and understand how many LEDs can be hooked up to an Arduino.
Tom Igoe wrote a blog post about frequent student projects, one of which was sensing touch and LED fields, however, I could not find it.
Flex sensors are like $19 so I am not sure they are that effective. Could you somehow use the optic/LED as switch, the person's hand touching it creates a change in current that would then trigger the LED code to turn on, i.e. for 3 seconds then reset?
I have used the flex sensor and they would work to a degree, but some of the fibers may push other flex sensor around, so I am not sure how well it would "follow" your hand. And it would cost a ton and a half.
As for LED as a sensor, have you seen this?
You may also consider using standard analog ambient light sensors, since that is what you want the LED to do and the sensor is actually made for that job, so it will do it better. Also consider if you want to run this is a room with the lights off .. you don't have any light to "read". So consider the LED brightness and ability to see them in a lighted room specifically for this experiment.
You can run lots of LEDs using shift registers, which is another topic .. you may consider starting there and seeing how many you can control with no sensors .. then play with sensors at the same time .. then put it all together.
If you want to control lots of LEDs in a small space .. see the matrix (not the movie).