Lesson learned, and thankfully my brother's hands and face are still healthy.
To get it out, I disassembled the gun, put the barrel breech-end up on a padded cloth on my garage floor and tapped the dowel with a hammer. The slug remained intact, and luckily so did the dowel.
Update. I took it to the range yesterday and cycled a few rounds through to make sure the action worked okay. Didn't seem to have any problems, so I fired a magazine of JHPs through (since I hadn't yet tested this gun with anything but ball). First round ejected, second round failed to feed. I dropped the mag, reinserted and fired off the remainder. Had a few more failures to feed out of the 50 rounds I put through. No visible damage that I can see, and I haven't had trouble with this gun before. Any ideas?

