You can absolutely get continuous length control out of FRC-legal pneumatics, with a little fiddling.
I put this together in 2019 (I apologize for the video quality, this was just something I was sending over to @Ryan_Dognaux and not meant for a giant public showing or anything. I also apologize for my voice and accent–I am from Western New York, and I’m not quite sure why people don’t punch me in the face every time I talk just to get me to stop. ).
I believe you can get smaller/cheaper valves that will do the job, but we got a big discount on these so they’re what we used. As mentioned above, there’s not a lot of strength in the intermediate positions, so you’ll have some fun programming a dynamic pressure control to keep the position constant in real time. (Alternately, you can have the pneumatic on a rack-and-pawl system so it will mechanically lock in place unless activated, which is another fun bit of design).
Ultimately, I think you’re almost definitely better off using multiple pneumatic cylinders, linkages, etc. to get a few desired positions, rather than going for continuous position control. But it is fun to play around with in the off-season.