A few people have done similar things. Technical pens are easy.
Syringe dispensers controlled by air pressure are a common way to go as well. Beware that there is a response time for air pressure to turn on or off, particularly with viscous or thixotropic substances. (You can add air pressure to the channel, but it takes time for the substance to move.)
One good precedent for glass capillary tubes is that some technical pens use sapphire tips. Beware that glass does wear down; paper is abrasive.
There is a “secret” provision in the software for controlling a digital output pin that is high whenever the pen is commanded down. Open up axidraw.py (in the Inkscape or CLI or python API) and look for
B3 in the code. It appears on three lines. Uncomment those lines, and then digital I/O pin B3 will be configured this way. B3 is set up as a “servo connector” with ground, +5 V (very little current capacity), and the I/O pin. That’s the highest set of 3 pins above the regular servo connector.
You can disable the servo moving by unplugging its cable or setting the pen-up and pen-down positions to be the same.