Yes, I've broken one (while sailing). The casting is easy to remove, you just drill out the rivets that hold the outer casting and remove it. Then drill out the two rivets on the bottom of the crossbar and the inner casting will come right out.
You can try removing the broken bolt by putting the casting in a vice and using vice grips on the bolt. But if the bolt is frozen/seized (which it probably is), then I suggest taking it to a machine shop and having them extract the bolt. That's what I had to do. When you reassemble everything, be sure to use anti-seize grease on all the bolts.
EDIT: Sorry, the above is how to replace the REAR inboard crossbar casting. The process for the front is basically the same except that in order to remove the outer crossbar casting (which you must do before removing the inboard casting), you have to remove the dolphin striker nuts so the dolphin striker rod can slide free.
This can be a bit of a pain if the striker rod is seized/corroded to the castings. It can be taken apart, but isn't always easy. If you can pop the dolphin striker rod off of the dolphin striker post, it usually makes things easier.