I’m not 100% certain, but it seems that having Eeschema open and hitting “Ctrl+e” will bring up the Symbol Editor; should be able to save to whatever library after that.
Thanks very much for your responses. I was just about to go down the text editor route…
Ctrl-E did indeed bring up the symbol editor with the rescue library highlighted. I was able to open the rescued symbol and do a Save As… to my library.
This is probably the best way to recover a symbol to your libraries. Just copying the entry in the .lib file won’t include the documentation information that is normally in the .dcm file. Doing the save as from within KiCad should pull the .dcm information out of your schematic to save in the library .dcm file.