I have seen the posts but have never seen nor had to repair such a problem.Possibly someone else will help out or request more information.

Looking through plumbing supply books I found tub drains which screw down into the tub drain pipe.If you look down the tub drain and can see threads then you may be able to replace this piece.The most common configuration is a solid pipe from the overflow down to the tub and down to the main drain.To replace this is not an easy job unless you have good access to all the tub connections both under the tub and in the wall behind the faucets.This may be why those $.35 drain stoppers sell so well!

Needless to say, there is a tool especially made for this problem. The cost and knowledge of its use would suggest, however, that you call a plumber. After all, replacing the gasket on a new drain is vitally important to avoid and future leaks. Best solution this time, call a plumber.

I am in the same predicament, trying to replace the tub, but having to remove the drain first. So far no one has offered any solutions. Since damaging the tub is not an issue for me, the ol' hammer & chisel trick is a real possibility...

Removing the drain is simple but care has to be taken.

All one has to do is carefully hack saw the tub outlet being careful not to scratch the tub or cut into the tub outlet. (cast iron)

The key is cutting it in two places using a sharp chisel knock out the cut piece and cave in the the threaded piece remove it and install a tip toes type of waste or the same make as the type you cut out.