Barbara Horgan has written me about the Balanchine Foundation's efforts to videotape coaching sessions - original Balanchine dancers coaching younger dancers - so the Balanchine interpretation is recorded and his original intent preserved. But it's very expensive and so she is searching for funding. To have videotape of dancers like Verdy coaching Emeralds would be wonderful, and I have seen tape of Allegra Kent coaching Darci in Sonnambula years ago. Miss Horgan wrote that she is working against time, as the older dancers, an irreplaceable resource, are dwindling.
Videos are definitely important. I was thinking however of written accounts by observers of these sessions. Several have appeared in DanceView in recent years, I believe. (Unfortunately I recently gave away my archives of DV and similar serious publications, and cannot recollect the specific works.)
There is a video of Verdy coaching dancers of the Paris Opera Ballet--it's quite amazing and, I believe, commercially available (edited to add: but it's of Liebeslieder, not Emeralds). But Bart is right about the importance of written accounts. I would add that I think more extensive scholarly work should also be undertaken to capture all of these voices in a systematic way.