Integrating these extension points will be possible once AsciidoctorJ 1.5.0 is released. Adding a plugin will just be a matter of adding the extension JAR (such as asciidoctorj-diagram) to the classpath. Asciidoctor won't support filters for AsciiDoc Python. Instead, you need to use the extensions that are built for Asciidoctor and tested with AsciidoctorJ.
Alex has several examples of writing extensions for use with AsciidoctorJ. In fact, it should be possible to take one of these extensions and test using it with the Maven plugin today (he may already do this). See the following repo for a collection of extensions that work with AsciidoctorJ.
You can read about how to use one such extension on Alex's blog.
I'm not sure if Asciidoctor Diagram (which provides the plantuml integration) has been tested with AsciidoctorJ yet, but in theory it should work. Anyone got a sample project workking?