I think putting it in unsupported would be fine. If some people (end
users) decide they really must have it, then more power to them to
maintain it in the aur (with a note to not have it taken into
community).
At that point it would just be a pkgbuild anyway.
Somehow, this doesn't seem very future proof. When a new codec arrives, it will take some time for VLC/ffmpeg/xine guys to come out with the open source equivalent (for example, the amount of time it took to get .rmvb support) after all the reverse engineering and voodoo magic they do. What if this new codec becomes really popular, really fast? Somehow, it doesn't seem practical to direct users to AUR to get the codecs. I propose instead renaming 'codecs' to 'codecs-nonfree' or something similar, and shipped along with the post_install or post_upgrade message we had discussed earlier.
Thanks,
Varun