Technically, scat applies to any excrement, feces or otherwise; after all, it's still coming from the anus. Though cum in ass isn't scat.
Scat is too common of a porn fetish term to change the name. It can't be as simple as "shit" because people will use that like poop is meant for.
There aren't many scat posts on the site, but I wonder if you'd be flamed for tagging non-poop scat (e.g. jelly) as scat? There's also defecating. Maybe you could add sub-tags for that one, like defecating jelly? In the meantime, defecating -scat. Though defecating also has posts that don't show the anus. Sure, you could add anus to your search, but maybe you should do something like:
This might be best if there's no existing tag for stuff coming out of the anus. After all, there are lots of scat posts where the poop has already left the body (but still fall under the fetish).