grails.gsp.view.dir to be array of dirs combined creating composite directory

I'd like grails.gsp.view.dir to be an array or set of dirs that are overlayed to create a composite virtual directory. My use case is that we produce a product to customers (grails war) that is often customized and extended. Not just style or content, that's easy. But, actually new pages and new functionality. For this we'd like them to have their own grails.gsp.view.dir like /opt/product/custom/grails-app/views/... We can do this now, but have to either copy our deployed gsp's to their folder, or copy their gsp's to the deployed server. For now the best option is the latter, but that requires server configuration changes and/or sym links that aren't as elegant of a solution.