I did a bit of research, and played around with the plugin code, but as far as I can tell you can't. You can of course set a default for ALL routes via "DefaultContentType" in the ServiceStack config (line 703 in the REST API plugin).
Try posing the question
here, or maybe if you search those forums you will turn up something I missed.
EDIT:
One thing about mythz at ServiceStack is that he is usually pretty responsive if he sees a good use case for a feature request.