[MadMax]

Maybe a hole could be drilled in the side of the gearbox. Run the motor at low voltage so it can't drive enough torque to overcome the preload in the mainspring which would define it's stop point so the peep hole lines up with the hole in the gearbox.

With the port in the gearbox and peep hole lined up, blowing out goo could be done with a duster can straw. Strategically placed holes could also be used to access the optoelectronics. A pair of opposing holes (on opposite box sides) above or below each sensor could provide direct access to sensor faces. Perhaps a bent shim of metal with a pad of felt glued to the inside face could be used to poke through the window and wipe the sensor face.

Maybe an entirely new lubricant could solve the gooing problem. Maybe painting on a coating of colloidal graphite could provide non gooey lubrication that would be easily blown off of sensor faces. Thrown off powder would be too small to occlude gear holes too.

I do find it a little goofy trying to detect a small drilled peep hole. There are lots of small processors that could count the sector teeth to derive sector position. The teeth profiles are nice and fat which make for unblockable optical flags. Also, with the teeth turned away from the sensors, a nice big gap is left to access the sensor faces with a Q tip through a hole in each gearbox side.