Thanks for investigating this! Yeah, it seems obvious in retrospect that async Comet communication is not possible without implementing the container-specific interfaces. Unfortunately, this is probably not something we can do in VaadinServlet due to the extra dependencies introduced.
However, a set of server-specific VaadinServlet subclasses might not be a bad idea. It could be an addon or even included in the core - in the vaadin-push package to keep dependencies sorted.