it is possible to set the theme for the page:
in JBP 2.2 you would do that via page properties, like:
<page> <page-name>Secure Policy Config</page-name> <property> <name>org.jboss.portal.property.theme</name> <value>Nphalanx</value> </property> .....
Note: the property name is subject to change (i.e. will be changed very soon; before the public beta)
Ok, I suppose it's not possible in 2.0.x then. I had a look at 2.2 the other day but it throw too many exceptions to be usable still, but it looks very promising for the near future.
Never mind, I've figured out how to accomplish what I need with AJAX.