Here's the design I made and tested yesterday. (It does something but not as well as I had hoped)

This picture is only missing the coil (the coil really blocks the view):
DSCF2849.jpg

The magnets push an aluminum tube that slides into a tight fitting larger aluminum tube. When they are overlapped enough there is a hole in the outer tube that gets blocked, closing the valve.

Closed:
DSCF2850.jpg
Open:
DSCF2851.jpg

The tubes fit surprisingly well but do not give you a full seal when closed. I was expecting that with a difference in restriction between open/closed, I could tune down the flow with a second valve until closed means almost no drip, and open is a decent speed drip.

As it turned out, when I turn the other valve down to the point where the closed position stops dripping, it doesn't get much more drips on the open position. I had to leave the other valve completely open and adjust the height of the reservoir until I found a spot where open means a fast drip and closed means a steady drip, barely on the slow side. It might be useful for some prints, or maybe there is a way to tune it better. But for now I'm looking at other designs and I'll keep you guys updated.