we would like to add additional options to advanced search, and we did so as explaned in the Wiki, except that when we selected a custom attribute to search against, it decided to show it as a drop down choice while we are interested in a multiple choice component.
Search in the web client for Staging or a User sandbox is only supported in our current 2.9 Community and upcoming 2.2 Enterprise release. That search support only provides full-text search support and does not expose the Advanced Search dialog found in the general ECM repo. I recommend reposting to the Web Client forum, where someone in the general Alfresco ECM community may be able to assist.