With the changes made to support BZ 457316, the web UI now offers some channel filtering options for package searches that aren't fully supported by the XMLRPC APIs for this functionality. This BZ is to address if those changes should be added to the APIs as well, and if so to track that they get done. It's not a trivial (i.e. 30 minute) fix, so I filed a separate BZ to let it get triaged for 5.3 rather than just making the API change as I updated the web UI.
I believe this is a dupe of bug 457350 which added packages.search.advancedWithChannel and could be closed as such.