However, it turns out that this never leads to an evaluation of extTyNamesLoc. I've come as far as to see that, since "Located a" is a synonym for "GenLocated SrcSpan a", the type we're looking for really has a binary constructor, thus we should use ext2Q.
This made things work: We first modify the function we apply to Located things:
extTyNamesLoc :: (Data loc, Data a) => SrcSpan -> GenLocated loc a -> OccurrenceTable
extTyNamesLoc l (L l' x) = case cast l' of
Just l'' -> extTyNames l'' x
Nothing -> extTyNames l x