The mechanism is 40+ years old and will not come out so that it can be replaced. Tried clr, wd-40 and brute force but it will not budge. Want to abandon it in place and put an overflow plate over the hole. Can I do that? The drain is open - it froze that way. Have been using a rubber stopper and want to install a pop up drain mechanism.

Even though the barrel is seized now it could loosen up and drop causing the tub to not drain, you better off removing the trip waste piping and trap then install a new pvc trap and pop up or trip waste… yes they do sell a 2 hole cover plate and gasket, it will be harder to find a pop up to fit the existing drain shoe

I'm having the same problem. I have a clogged pipe & while plunging, the trip lever went into the closed position.
I tried to pull the lever out through the overflow and broke the top part off. I cannot access behind overflow without cutting open a bedroom wall. Any suggestions on how to break the one-piece trip lever that goes vertically and horizontally? It's in the closed position so tub water wont go down and sink water is filling into tub now, too.