Is there an aftermarket device (or home engineered one) that extends the radius of the fine focus knob for very fine increments of the focal plane?
There certainly is. Between 'stiction' and the small size of the knob, it is sometimes desirable to have an aid in incrementing the knob, as for focus stacking. Here's what I do:
Say for example your fine focus knob is about 1/2" (12mm) in diameter. Get two pieces of wood or plastic, each about 10" (250mm) long and 3/8" square (sort of like a couple of long chopsticks). Fasten them together at one end with string, wire, or tape so that they are about 1/2" apart there, but are tied together pretty well. Put this arrangement over the knob and down near where the sticks are tied together. So now you see that if you squeeze the other end of the sticks together, they will squeeze the fine focus knob like a nut in a nutcracker. Put a rubber band over the two sticks to hold them on the knob. To change focus, just squeeze lightly and rotate the end of the two sticks (and thus the fine focus knob) through an arc the desired amount. For focus stacking, use a small increment, e.g., move the end of the sticks about 5mm or so, take an exposure, move again, etc. You could even set up a piece of cardboard behind the sticks with an arc pre-divided into increments.