Just came across this thread as I have the cracked nut problem too...the one on my rod that is.
I first glued it with JB weld and let it sit for a couple days with a clamp holding it in place. Then I took heavy waxed whipping thread and tightly wrapped the locking nut multiple times, especially near the open end where the reel foot slides under it, whipping it once I got to the end of the thread. I put on 5-6 separate pieces of thread each time with about 3 feet of thread each time and now it seems solid. I am going to try to find out what finish, polyurethane, shellac, etc. adheres to waxed thread and coat it to strengthen and protect the wraps and lock them in place. I mounted the reel back on the rod and so far this seems like it will work. Have not fished it yet.