It would be an interesting test to install a greasable joint in a driveshaft, but first, drill and tap each cap and install 4 grease fittings on the joint, just to be sure grease made it into each cap. Maybe the joint would last forever, maybe it wouldn't last any longer than a non-greasable. Who wants to try?

 

Are the caps thick enough to tap?

 

yes, they are- some joints have a hole already in the cap on one end for limited clearance. I've seen some with both body and cap locations- 1 zerk in the package and a plug. In fact, the one I put in my front drive shaft many years ago had a flush needle type zerk in one cap but I could never get the needle to go in so I would unscrew it and put in a regular zerk when greasing, then put the other back.
I think I have a spare of the joint on the bench...