Webkit's stance is basically "we're only doing it if upstream libpng supports it" (because APNG support basically requires a couple hooks in libpng itself) but the libpng devs refuse to accept the patch for apng support because they are of the opinion that libpng should process only PNG, not derivative formats, which is what they consider APNG.
No need to fork libpng. Currently, vanilla libpng and apng patch are both hosted on sourceforge, and it's trivial to automatically download both and apply the patch. In fact, a few distros (Arch, Gentoo, Mandriva, FreeBSD) already do exactly that for their libpng packages.
But keep in mind that apng patch only adds a few functions to libpng API, and it's entirely up to applications to actually use those API calls to request extra frames.
