The REST api should expose the keywords found in a topic as an optional, expandable property on a topic's JSON representation. This would open up the possibility of matching topic together, as well as provide a way to match topics to other external content.
This has been deployed as part of the 1.3 hotfix. Expand the keywords property to view the list of topic keywords. StringConstant 73 is used to define stop words (i.e. words that should not be keywords) IntegerConstants 2 - 6 define the variables that determine how the keywords are generated. They map to the settings in the Lucene MoreLikeThis class (http://lucene.apache.org/core/3_0_3/api/contrib-queries/org/apache/lucene/search/similar/MoreLikeThis.html). The constants are read at runtime, and so can be used to tweak the generation of the keywords in real time.