As of the current version of Grails we are using (2.0.1), the grailsApplication bean is not available for use in the UrlMappings class. However, the UrlMappingsHolderFactoryBean implements GrailsApplicationAware, so we can call getGrailsApplication() to do things such as base or mappings conditional on the configuration, like as follows:

class UrlMappings {
	
	static excludes = [...]

	static mappings = {
		if(getGrailsApplication().config.doA) {
			"/**" (view: '/a')
		} else {
			"/**" (view: '/b')
		}
	}
}