I believe this is the same request as
https://github.com/asciidoctor/asciidoclet/issues/42. If the doclet can export the Javadoc string to disk, then it opens up the door to combining it with other sources using the existing Asciidoctor toolchain.
Where we are with that issue is that we need to pin down some design requirements so that it's possible to move forward with implementation.
Cheers,