Is it possible to extend nsIDOMNode and alike?

I have Mozilla embedded in my application through Java-xpcom. I want
to attach certain reference (to Java object) to each of the nodes in
the DOM tree. In another word, once I get a nsIDOMNode from Mozilla, I
would like to call methods on its attached reference. I can certainly
build a map and store the references in it. But it won't scale well
and constantly adding/removing from a big map is kind of expensive.
Since nsIDOMNode is an interface, I am thinking if I can wrap
Mozilla's implementation in my own class and somehow provide that
class back to Mozilla DOM then I should be able to achieve what I
need. The problem now lies in how to provide the class back to
Mozilla. nsIDOMDOMImplementation looks like a potential entry point
but is read-only on nsIDOMDocument. Is the route I am taking even
possible in xpcom? Are there any other solutions that you can suggest?
Thanks in advance for your help.

Feng