Actually, you're right. God, what was I thinking. The only potential problem is that implementations that DO implement callcc across ffi will wind up with code

Actually, you're right. God, what was I thinking. The only potential problem is that implementations that DO implement callcc across ffi will wind up with code that won't run on implementations that DON'T. That would be bad, but not the end of the world. The real problem is that it's unlikely such a thing would be standardized.