You're right, I didn't notice it, nor have I ever heard it brought up in this context, actually. McGonagall and Trelawney appear to dislike each other, but we don't know why -- if they had the same kind of known history that Lupin and Snape do, maybe more would have been made of it.

This is largely why I asked the "what else could Lupin have done" question. Snape is obviously put out by their interaction, but as everyone should know, he's oversensitive about being disrespected and embarrassed, particularly by MWPP or anyone tied to them. So while I still think Snape felt needled by the first-names thing -- and I still think Lupin knew that -- I also think Snape would have found plenty of reasons to be annoyed besides that even if Remus had taken the hint.