Is it possible to detect a keyboard paddle press plugged in the headphone/microphone port? I know that some devices headphone ports (maybe all of them? any reference here would be good) have 3 channels, for stereo sound and microphone. So I was wondering if it's possible to detect something like this pedal on these devices. 

If it's possible, using the audio recorder would be enough? I'm studying possibilities for an app here, and this sounds promising if possible.
Sure you can, there's a product that uses the audio port as an extra button, check this:

http://gizmodo.com/add-a-button-to-any-android-phone-with-this-genius-head-1221431712

Maybe this is more like a comment than an answer, but actually i can't add comments untill i reach 50 points.

By the way, as microphone it's an analog input for the android, you can set a trigger point on certain value to detect an (On/Off) state, for example:

If microphone values are in a range from 0 to 1024, you can set that if input it's less than 100 consider it as Pedal Pressed, if it's greater than this, pedal it's not pressed.

You just need to read microphone input and convert it to it's analog values instead of treating it as audio.