In article , "andrewpreece"
says...
Aside from brute force and the rotate-and-pull method, I was wondering what
else might work. I reckon if you're really stuck ( ha-ha! ) as a last resort
you
could manufacture a hollow cone out of something suitable ( wood, foam ? ),
and slide it flat end first down the rods. The problem is getting it down to
the
flate plate, though a torrent of water might push it along. Once at the
flate plate
( which you push against the far wall of the main branch whilst doing
his )
you are then free to pull the assembly back. The cone should allow the flate
plate to ride up over the lip of the domestic/main sewer intersection.

It doesn't need to be a cone - just a cylinder long enough not to
fall into the pipe that the plunger is stuck in.