Before doing so, I want to ask if anybody is willing to step up and maintain these loaders. Note that even if we drop these from gdk-pixbuf itself, they can be maintained out-of-tree... one of the advantages of having loaders as modules.
Not stepping up to maintain those, but I really like Emmanuele's idea of splitting the other modules into a separate repository on git.gnome.org; I think there is value in keeping them all in a central location.