If the striker rod is frozen to the end castings, the only option besides liquid wrench and hammering that I can think of is to drill out the rivets holding the mast step to the front crossbar (two rivets on the old style, four rivets on the new style mast step). This will allow you to lift out the mast step and dolphin striker post. The striker rod will still be in the end castings, but with the post removed, you may be able to knock one of the end casting out of the crossbar. If you can get one of the end castings out of the crossbar, then you should be able to put a pair of vice grips on the striker rod and spin the end casting to break the rod free from the casting. I know I did this once, but I don't recall if both end castings were frozen or if it was just one. With the striker post removed, you may also be able to break the rod free by tightening the nuts on the end. Anything that allows movement between the rod and the casting will help get it apart.
Obviously, the last resort, if you really need to get the thing apart, would be to cut the striker rod and pull the end castings out. Then replace the striker rod.