It would be tedious, but grinding the pin with a die-grinder till it's just a shell would allow you to collapse it and possibly get it out.
Also, the welding idea is worth a try, and probably wouldn't heat the crank enough to damage the oil seal. If you wind up removing the crank, you'd have to replace the seal anyway.
Some pins I have seen are stepped (smaller diameter in the crank than the part that goes into the flywheel or other mating part.) that would prevent them from being pushed into the flywheel like happened here.
