You can hear the song here:
http://www.youtube.com/watch?v=oKLS6oBWsYY
If you listen carefully, you can hear Glenn's and Don's harmony on the third verse... "Love was here today..."
I think it's lovely but no, it's never been performed live to my knowledge.
Edited to add: I just saw a comment on YouTube where the guy says he DID hear it live as an encore!
BTW, regarding Randy needing an extra vocal, it's interesting that he got three on the album but Don only got two. I know it's been said that Don and Glenn were teamed up from the beginning, but I think that didn't happen until Desperado. I think at first, Glenn and Randy were the pair, as were Don and Bernie. Randy sang on Glenn's song, and Don and Bernie co-wrote Witchy Woman (and both had two vocals instead of three). This is borne out by Glyn Johns saying that Randy and Glenn were always sneaking off together to do pot, lol. (I do realize that Randy co-wrote a song with Bernie, too, but I'm just following an overall pattern).