I too have often wondered what did Darcy say to Georgiana and when did he say it. You've provided a very plausible explanation.

I will only add that whenever he spoke to Georgiana about Lizzy, he would have been careful not to do it in front of Bingley and the Binglettes. He would not have wanted to draw Mr. Bingley's attention towards Jane by speaking of Elizabeth. And he would not have wanted to invite Miss Bingley's witticisms about her 'fine eyes'.