Can I drill into the cast iron cap on the clean out trap to remove the nut?

Are you the same person that has a CI cover stuck in the sewer pipe or are there two of you? Back to you. Tom

Should we assume you have already tried a large pipe wrench with no luck? I can't see how drill a hole in the cap will be of much help unless you drill out almost the same size as the plug. If a cast iron cap is that snug I'd get a sharpe cold chisel and start breaking the cap from the center, once you chop out a large hole then you can collapse the plug by hitting from the side driving the O shape into a C shape