Mechanical drag from guiding the filament isn't really that great. Bowden extruders push through a long guide tube, and I pull through one on my MG Prusa. Furthermore, I added a printed guide tube at the end of the Bowden tube that turns the filament (1.75 mm) through 90° of a ~35 mm radius as it comes off the spool. Granted, I couldn't get away with that tight a bend on 3 mm filament, but my point is that it is possible to guide filament without putting too much drag on it. Placing the sensor right at either opening of a Bowden tube (or similar) should constrain it within about 0.5 mm of the desired location without adding all that much drag.
Spitballing other ideas: To constrain the position further, 3 bearings spaced 120° around the filament may yield a position more consistent than your rig can currently measure. (The OpenRC crowd may have some size suggestions for cheap bearings of an appropriate size.) The bearings couldn't all be at the same point along the length of the filament, of course, unless one of them was on a complaint mount of some kind. Some other arrangement of bearings could allow the filament itself to provide the force that keeps it against all the bearings, but that might require more bearings. I'm imagining some all-printed scenarios too. Just please don't give up on guiding the filament for fear of drag. It's quite manageable. And +Tom Haynes
's idea is certainly a viable option too.
Great work so far. We need this!