Norm stated that this might be of interest to the publishing committee: http://sf.net/support/tracker.php?aid=3383019
It is possible to put it any Music XML content in the appropriate namespace (parallel, for example, to the way DocBook includes SVG and MathML).
There will likely not be any DocBook-specific elements needed to support Music specifically.
Based on the Publishers SC meeting on 15 Feb 2012, we will not be adding any new DocBook elements to support this request. However, you should be able to extend your own layer to include Music XML content in it's own namespace similar to SVG and MathML. We will try to publish some guidelines on the DocBook wiki on how to do this.