This is just speculation, I haven't tried it myself having not yet delved into the programming to activate it:
Has anyone tried the guts of a PC-mouse as an encoder? I'm thinking a mouse with scroll-wheel (most have a mild 'detent' feel) - perhaps an old(er) ps/2 plug type? Use a ps/2 socket to retain the 'foolproof' pluggability. Disconnect the wiring (maybe leave supply + and -), and wire out the opto's of the wheel. One will be SDA, the other SCL. Perhaps also a/the button/s? one as encoder's 'push' menu/function for step-change etc (I'd use the right for that), the other (left being more intuitive for the more prevalent dexterous :) of the populace) for a convenient PTT... Note: will they pull up or down?
That's 6 wires (if the PS/2's connector pins are all wired through the cable - they might not for a mouse) - SDA, SCL, +5V, 0V, menu and PTT.
If your scrapped mouse is a 'track-ball' type, remove the ball to remove the temptation from the kids to swap it to their old 'junker' PC. Pop in a large nut from the garage/workshop's junk-heap to lend weight if required.
Feedback? Flames? Go to it ...