Wet return rotten where it meets copper

Single radiator branch's 3/4" wet return from a small addition built in the 50s. I realize copper to black pipe is not ideal, but it's been running this way for at least 15 years probably 30+. I replaced a similar junction a few years ago at the rear of this picture. The new problem is in the foreground. As soon as heating season is over in a few weeks here in NJ I thought I'd heat up then nearby union and replace the rotten nipple.  Can you use a dielectric joint/union here like you would on a hot water heater?  Other recommendations?  Otherwise I guess I'll just replace the rotten Piece with black pipe nipple and wait until it rots in a decade.