There's a discussion from back in 2008 on a talk page that I can't find again, where an editor asked about the very concept of organizing by source text in relationship to fans who are fans of the fic for a pairing and don't care about the canon at all.

I was also having a discussion on my journal about the idea of fans intentionally seeking out characters to slash, rather than finding a source slashable. And I feel that fans do both of those things for fandoms that other fans access via the canon.

I find myself amused/interested by the idea of having a "Source material: fanon" category for fandoms that are particularly non-canon-regarding in this way.