You can define custom output visitors and tag it (name: ezpublish_rest.output.visitor, regexps: something) but it won't work with current implementation unless you load your bundle before new eZ\Bundle\EzPublishRestBundle\EzPublishRestBundle(), (and this way sucks). It's because of order of output visitors passed to compiler pass while kernel configuration contains Pokemon "gotta catch'em all" regexp (^./.$) so it always return application/vnd.ez.api... regardless of visitors and generators registered after that one.