I know that the Text Encoding Initiative's guidelines define an XML vocabulary for non-technical text (such as poetry, fiction, etc.) but would like to stick to DocBook if possible, even though it has no XML elements for dialogues, poetry, etc.
Chapter 5. Customizing DocBook in DocBook 5.2: The Definitive Guide explains how to add custom elements and attributes (among other modification possibilities).
There is however an official subset of the DocBook 5 XML vocabulary for non-technical writing, and it has it's own book: DocBook Publishers: The Definitive Guide. (It is only mentioned on the "Documentation" tab but not on the main page.) It already has elements such as poetry, drama, dialogue, line, etc.
A summary of the differences from the book:
Based on DocBook V5.0, the DocBook Publishers schema is normatively available as a RELAX NG Schema (with some additional Schematron assertions). Of the 361 total elements in the full DocBook standard, the Publishers schema has been simplified to exclude 149 elements from full DocBook. Modifications have been made to 11 patterns from the DocBook schema. Additions to the schema include: the Dublin Core metadata elements (54) and 6 new elements.
Subsequent chapters go into the specifics of these changes.