I don't know of any Eclipe plugin related to Jaxe.

It is possible to create a Java application that includes Jaxe as a textpane, and manage events (with JaxeDocument.addEditListener), [I'm not sure what you mean by behaviour - behaviour of what ?], change the view for elements by editing the config file and possibly adding plugins for custom jaxe elements, add a custom toobar... All that has been done before, for instance in this (unfortunately private, never publicly released) application: http://jaxe.sourceforge.net/infomanager.png
So, you can do all that, but things like managing events or adding a toolbar cannot be done with plugins: you need to create a new Java application (or applet) and include jaxe.JaxeTextPane (which derives from JTextPane) inside (look at the developer guide for an example). This means you will also have to create the menubar for the application, but you can take a look at jaxe.JaxeMenuBar if you want to do similar things to what is done in Jaxe.

Should I leave this "support request" open ?