Hey anon, I don’t think I have a definite answer to that and if anyone does and cares to share, I’ll be happy to post it here. I just think ‘Rhiannon’ is a song she feels very comfortable adding ad libs. And it’s just a song that never stops evolving.
“So the interesting thing about that song is that old witch Rhiannon, she has been in every step we’ve ever done since 1975. You just can’t get rid of her. She just puts her wings on and flies in and takes over my world…” (24 Karat Gold Tour, 2017.)
The first live performance of Rhiannon that we know of was during a ‘Buckingham Nicks’ concert and she tells the band not to play it too fast:
If I remember correctly, she once played a slower/piano version of ‘Rhiannon’ and said that was as it was meant to be played. I don’t remember what particular performance that was though.
I love how around the ‘Tusk Tour’ she started singing “taken by the sky” all through the song.
In this video from 77, she sings “golden, emerald, satin…”, which some believe might be a reference to ‘The Three Birds of Rhiannon’.
There’s a performance from the ‘Tusk Tour’ in 1980 where she sings “is that what you want?” several times and it’s equally fascinating. (Download here.)
Here’s an interesting bit from the ‘Enchanted’ liner notes:
“With all the bootlegged piano versions of ‘Rhiannon’ that are out there, I decided to just go into a studio with a grand piano, sit down, and play ‘Rhiannon’ to you, as she is today. I feel her wisdom is much evolved…and you will hear that in the piano, and in my voice. People say, ‘Do you really want to go through that again?” And all I can say is “It’s my pleasure….”