Wanted to let you know that I am experimenting with adding support for this in trunk (-> 2.2.0).
There are 2 options in play here:
1) <options><applyStandardInjectionEntities>true</applyStandardInjectionEntities></options> (true is the default). Makes some "standard" information available to your docbook source in the form of DTD entities. Currently there are just 2: (1) "version" which is the defined version from pom; (2) "today" which is a new Date() formatted as "MMMM d yyyy"
</value></entity></entities> would again make available a DTD entity named "version" available to your docbook as a DTD entity with the value of your pom-defined version.
I think this should allow coverage of most cases.