DWR automatically converts DOM trees from DOM, DOM4J, JDOM and XOM. You simply return a Document, Element or Node from any of the above and DWR will automatically convert it into a browser DOM object.
There is a common warning on startup about the JDOM converter, which you can safely ignore (unless you wanted to use JDOM):
INFO: Missing classdef for converter 'jdom'. Failed to load uk.ltd.getahead.dwr.convert.JDOMConverter. Cause: org/jdom/Document
The problem is that DWR has no way of knowing if you wanted to use JDOM or not, so the message is at INFO level.
If you wanted to use JDOM you might be very pleased to know that the converter has not been able to load - which is why we've not taken the message away.
I believe that DWR will work with exist-db out of the box, because builds on top of W3C DOM, which is supported.