No one knows? :((
you can set the cache in the portlet.xml (descriptor of the portlet).
unfortunately, if you don't want to remove the cache feature for all request, but just in a few cases (as in your scenario), there is no way to do that.
That's what I know for now, and concerning the JSR-168.
I am looking at how to invalidate the cache for a special portlet, at a given time (and programmatically)...
I guess that can be done with specific implementation of cache for the jboss portal.
hope that helps...