It's probably not considered canon. I think they conceived the idea of the Celine and Jesse scene prior to conceiving the idea for Before Sunset. I don't really know though.
I think you're probably right, although they obviously knew the scenes in Waking Life existed when they were writing the script for Before Sunset--so they had the choice to follow that continuity, or not.
They are named when you look on the imdb page and it's a dream (as the entire movie is). It may be a dream from Jesse or Celine or from the main character in this movie or it might be a dream from us (the audience). Considering this, it is canon. They probably did not tink about Before Sunset when they made Waking Life but it fits with the continuity.
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You see things; and you say Why? But I dream things that never were and I say Why not?
It's also a scene in which they are in total agreement without a hint of an argument. I too think that it's not part of the canon but certainly a realistic what if. It is the only time we ever see Celine and Jesse in bed. ------------------- Suspension of disbelief is a privilege, not a right. Abuse at your own Peril.
I concider the moment a meta-moment, I think the main character of WL has seen before sunrise and as sometimes happens, the characters of the film he saw appeared in his dream. Therefore it is not canon in my eyes, it is just a part of a dream.