I finally got around to removing mine, and here is what I did: I drilled four holes, one at each corner of the plug "nut". I let the water drain out, then I cut out the nut using a metal cut-off wheel (Rotozip).
After this, I used the cut-off wheel to slice away portions of the cap until I could get near the inner ring. I then cut slots right up to the edge of the cast iron pipe.
From there I used a hacksaw to cut vertically from the inside out to the edge of the threads. You can feel and hear when you start grazing the cast iron, so cutting by hand this way is safe. Once I'd cut through the plug in about four places, I tapped the chunks out with a cold chisel.
I don't have a replacement plug yet for the odd 3-3/8" cleanout, but I'll go hunting today. I was amazed at the amount of crud in there -- a big plug of junk and sediment lining the walls of the cleanout.