Web Deployment Descriptor task

[Tony Morris]

I'm looking for an Ant task (or any feasible solution actually) that would modify an existing web.xml deployment descriptor. Id like to solution to not require changes to the original web.xml file.
An ideal solution would be a <webxml> task and would look something like this to add a servlet-mapping to the deployment descriptor:

<webxml dir="blah/WEB-INF">
<add>
<servlet-mapping>
<!-- blah -->
</servlet-mapping>
</add>
</webxml>

If anyone knows of such a task, or has a solution to this kind of problem, I would appreciate learning about it.
Kind of like AOP for web deployment descriptors
Thanks.