First things first. Make sure there is no ammo in the magazine tube. As others have said, soak it in Break Free. As the name implies... well if you haven't caught that yet, it'll be hard to explain now.
There is no gun that is rusted so bad it can't be opened up. The Mossberg is no exception. You can punch the takedown pin out of the trigger group and pry that out. Take the stock off (rust on one side only as the other side is aluminum). I've got a big sheet of 3/4" thick rubber so I'd take the gun without the stock and, holding the forearm, wang the back of the receiver against the rubber. Not hard enough to break anything, but it should open up enough to get the bolt carrier and bolt out with that treatment.
If it doesn't, you can insert a rod down the front and whack at the bolt itself a few times.