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.