Wheel grease wise I always drill a small hole in the center of the hub. If I can't get an official grease cert mounted there I at least have a hole to butt my grease gun to. I do this to all my wheels and I pack those suckers! Done it that way for years now before I new what motor assist was
It is neat to observe because you can see the old black grease oozing out the sides, and good grease starts oozing there just wipe off the access and ride on!!
Been a very along time since I messed with a coaster brake hub. I like the freedom to peddle backwards. Prolly want to disassemble those and clean them out?