I had an offer of plumbing help from Rex Frankel of Rare Earth News in comments below, so that has spurred me to post of picture of what I'm dealing with in the bath right now. This is photo of the pipe nipple that normally feeds the tub spout. As you can see it's original 1954 galvanized that has thoroughly rotted. I suspect that corrosion was helped along by caustic cleaning solutions that worked their way back onto the pipe. The tile is a homeowner special of indeterminate age.
Since I'm planning a repipe soon enough, this might be the goad that gets that whole effort going. However, because my son and I have to live in the house, I had wanted to phase the repipe in over time without disabling the bathroom or kitchen (which shares the wall with the bath) until I'm ready to deal with the consequences. Therefore it's worth looking at repair options.
Repair plan of attack:
1. Dig out caulking with tub plugged to prevent debris falling into drain. Use wet/dry vac to clean up debris.
1A. Inspect further back in wall.
1B. Apply Liquid Wrench to threads at elbow.
2. If it looks solid further in, attempt to unthread with pipe wrench
3. If that attempt fails or appears unwise, apply heat with MAP torch (don't have a lower heat propane torch). I believe the idea is that thermal expansion of the metal will break rusted threads free.
4. Attempt to unthread with pipe wrench
5. Measure depth of elbow in wall. Visit full service local hardware store for a basic replacement spigot and assistance cutting and threading new nipple to correct length. Maybe a stock length will work.