This would be a VERY useful feature, and one I'm surprised wasn't implemented with the CC feature, it seems odd to include the one and not the other to me. The ability to *easily* find images that are free-for-use would be a much bigger help against art theft than just being able to mark works as sometimes useable etc. It would also help people like me who are trying to find art that can be used in certain situations, instead of clicking images, scrolling, and hoping.
I support your suggestion 75%. The other 25%, unfortunately, I reserve, not because I don't think the CC Search would be a very useful feature on DA, but because DA allows custom rules to be added to the DeviantArt members' pages in addition to the rules built into the Creative Commons Licenses (e.g., on the deviation page, partly on the deviation page/partly on the stock owner's Profile Page, in a Journal, in a widget). Those custom rules, in many cases, conflict with the Creative Commons licensing rules.
If I run a search for a photo licensed as "Creative Commons - Attribution," I want to trust that my responsibility in creating a derivative work from said photo (outside, of course, the common restrictions against using the photo inappropriately) is to provide credit to the owner of the stock photo in my deviation description, as required under the CC-Attribution license. I do not want to post my work to DA only to be accused later of breaking one of the "custom" rules I often come across (e.g., can't use the stock photo for equine-related derivatives; can't use the stock photo for photomanipulations, only for reference; can't use the stock photo off site or in a banner; can't use the derivative work for prints without permission; can't use in a religious theme; can't trace; must remove the object from the background; must provide credit on the deviation itself -- the custom rules go on).
I've nothing against custom rules; indeed, I spend a lot of time tracking them down from their various locations to see if they in any way conflict with my usage intent and so that I can abide by all of them. I just don't like them used in addition to the rules set by Creative Commons licenses. I would start out using the Creative Commons Search, if DA could implement that feature, because it would save a lot of time in locating stock photos with common restrictions; but if the search results included a large number of photos with rule conflicts, I most likely would discontinue the use of DeviantArt's CC Search feature, and would probably continue gravitating towards using photos from Wikimedia and Flickr where it takes me only a few seconds to determine what the exact rules are for their use.
Custom rules that add restrictions are not legally compatible with CC-licensed work.
Flickr allows you to write whatever you want into the photo description too and I'm sure Flickr users use CC licenses and then add custom text prohibiting re-distribution.
Adding a license filter will make the CC choices more relevant and they will come to the attention of searchers and artists whose art is being found via that channel more often, in the end making dA users better accustomed to these licenses and how to use them as deviant or derivative-ant.
The search function would be hard to implement, I believe it's something to do with how the license property is stored. It's in the db, but not in the full text index. I can't remember, but I think they've got a modified cLucene index, I'm pretty sure they could enable field indexing.
Otherwise, perhaps creating a separate index for this kind of property. Creative commons license searching is CRITICAL and it's absence makes deviantArt look unprofessional. There is no way I can do my business of remixing works on deviantArt, the only thing that comes close is entries volunteered to the various groups.