Can the drag washer under the main gear be replaced with a carbon fiber washer?
Yes,
Delrin is a better substitute, but the carbon fiber will work.
Yes, BP --
It should be LIGHTLY greased so as to not interfere with the dog --
And of a diameter a little smaller than the drag washers -- so there is no fraying to the UG washer from the gear teeth. Circumference must be inside the bottom part of each tooth.
A better solution is a hard disc made of Delrin -- not greased, dry.
Best,
Fred