More useful than a struts-config editor would be an XDoclet mergefile editor. These files cannot be edited with an XML editor, because they are not well formed XML (they lack a top level element). The editor could figure out from the name of the file if it is a web.xml mergefile (servlet-mappings.xml), a struts-config.xml mergefile, etc. The editor could then look at the DTD for that fragment to generate the apropriate tab expansions.
Thanks,
Calin