Two-state clicker button
Ok, I have managed to build a working cam mechanism using LEGO Technic. Only thing missing is a big enough spring or elastic band to pull the entire assembly upwards - Still thinking through that part but sharing in the hopes that others can take my idea further (My current best thought is to have rubber bands on the outside of the mechanism pulling the "clicker" upwards.). Here is a demonstration of the mechanism at work (was difficult to demonstrate with just one hand as I haven't lined the walls of the vertical holder enough and sometimes the up/down motion goes under an angle): video on youtube.



Pictures:

The entire mechanism:


Clicker in the "up" position (i.e. retracted pen):


Clicker in the "down" position (i.e. pen out) - my hand is in the picture as there is no spring to provide pressure upwards:


The insert with the loop (I believe this is what is called a "cam"). There is a resting point on the top (the "down" position) as well as a resting point on the bottom (the "up" position):


The "ball" - in this case a simple Technic axle with 2 1/2-width bushings on it - is supported in this mechanism, which allows the black part to slide freely from left to right. This "ball" moves around the loop defined by the "cam" from the previous picture: