I'm not sure to understand :
You are able to overwrite locale.portal.webui successfully but why you said the overwriting resource bundler does not work ?
Your new resource bundle declaration are not in the locale.portal.webui ?
The resource bundle locale.portlet.web.GroovyPortlet resides in exo.portal.portlet.web, the resource bundle locale.portal.webui in exo.portal.web.portal. Overwriting the second one works, overwriting the first one does not work.
To bypass the problem I made a copy of the groovy template replacing the affected resource bundle keys with new ones defined in my own resource bundle.
I do not think GateIn supports to overwrite resource bundles of a portlet like that way.
As the loading mechanism of resource bundles in a portlet is that the portlet's resource bundle will be detected first then to the portal's one.