In the DAM search Content Finder tabs that appear in the left column of an edit view of a page, we want to be able to search by the Title of a tag as well as the Name of the tag. I've written a Filter that replaces the "query" and "query_term" parameters in the request, if they exactly match a tag title, with the corresponding tag name, but the jcr query that transpires still searches on the text I typed, not the substitution from the filter. Even though I've verified that the DefaultSuggestionHandler does look up the altered value of the parameter in the request wrapper.
You might have done at  but not at dam.assetsearch.json .
Not sure which version of cq & hotfix you are using. Plan to upgrade to latest where you have capabilities to extend CQ DAM search easily oob http://dev.day.com/docs/en/cq/current/dam/extending-cq-dam-search.html