The easiest way to do that is probably to use a new siteaccess with its own design. So you just have to put your xml tags templates in the right folder and it should work.
Another solution is to play with eZTemplateDesignResource API to build your own override condition. For instance, in your module before calling the template engine you can write something like :
André: I've noted something similar with my (unpublished) eZXMLOutputHandler override (it allows you to define different views that can be used as an override key - for example, to have a simple way to get a proper plain text string from an XMLText field).
For 4.1 I had to override an additional method, reintroducing the old DesingKeys handling - why exactly did you change that? Or is there a clean way to add a new DesignKey in a subclass of eZXMLOutputHandler?
Accessible website starting from eZ publish 3.0 (currently: 4.1.0): http://pluspunkt.at
