Hello,
I don't think that it has been done.
However, as you said, since a Graphviz plugin already exists, creating a PlantUML plugin should be easy, since the general architecture of both plugins should be the same. (Retrieving the diagram text, launching the executable, retrieving the image file)
I don't know where the DirectedGraphPlugin code is located. The only public file I've found is:
http://twiki.org/p/pub/Plugins/DirectedGraphPlugin/DirectedGraphPlugin_installer
I suggest that you post a request on some TWiki forum: creating a new plugin should be easy for a TWiki developper.
We could give some help if needed about PlantUML.
Regards,