Board Thread:Support Requests - Community Management/@comment-1376932-20130320001735/@comment-4674838-20130419021222

So far as I am aware, there is not. Special:UnusedFiles only lists a maximum of 1000 results in the cache, and only updates roughly once every 24 hours. That's just a software limitation one has to live with.

You could try contacting Wikia at Special:Contact to let them know that you would like more results to show up, or for it to update more often. I'm not sure if you'd get any traction, but it wouldn't hurt to ask.