you sure that isn't a flush style zerk fitting that takes a needle grease gun?

you could always take note of how it is phased, make a mark with a marker or whatever, and pull the slip out and put grease in by hand.. and reassemble it the same way it came apart

Nope, no zerk, I can see the spline through that hole. There isn't much depth for a zerk to fit in there either, I'm not sure what was there originally.
I thaught about greasing it by hand, that's how my brothers front driveline is. I'll have to plug that hole or find a zerk that fits, maybe I'll also stop by a driveline shop one of these days to ask about that screw on cap or whatever it's called on the slip.