Currently build-docbook-catalog does not support the Simplified XML DTDs (app-text/docbook-xml-simple-dtd), so they do not get registered in the XML catalog unless explicitly done (as only one version does). I'm attaching a patch for build-docbook-catalog-1.5 to rectify that.
Created attachment 208400 [details, diff] patch for build-docbook-catalog-1.5
Thanks for the patch, fixd now.