Thanks for the link to my blog.
Here's what I'd do:
use a dowel, cork or some other method to securely plug the chamber. Point the breech end in a safe direction and CAREFULLY use an air compressor (maybe use one of those plastic nozzles used to fill beach balls and such...whatever you use, be very careful not to bung up the crown) and pump some air pressure into the barrel to try to blow the piston out.
Start out at a fairly low pressure and work your way up.
Unless you've got some sort of super-duper compressor, you shouldn't be able to put enough pressure in there to hurt the barrel...but be careful of the piston or breech plug shooting out and ricocheting around.
You'll shoot your eye out with that thing.
Thanks again for the link.
Sailorcurt
Captain of a Crew of One