I used a nylon, 1/4"-20 machine screw with the threaded part cut down to roughly 1/8" long. I used a hack saw blade to cut the screw. I paid $0.45 for a pack of 3 screws at my local hardware store. Simply unscrew your broken lubricator from the hub and replace it with your modified nylon 1/4"-20 machine screw.
These socket cap screws would work great because you can use a long Allen wrench to insert and extract it. I'll have to see if I can find one locally.