postMessage is just one of the transports we use for Web Intent Events in this version. There’s also a Flash object transport for old versions of IE without postMessage support. We already announced our plan to deprecate (or partially deprecate) IE6 and 7, and being able to update the events architecture to lose that abstraction overhead is part of that decision.
However, because we abstract the messaging over multiple transports, the current payload we send is not part of the documented API (we currently use an implementation of JSON-RPC, fwiw.) This abstraction has an initialization phase, which relies on widgets-js being present in the parent page, and knowing the identity of the frames it creates.
The future implementation will not have any of that overhead. You could likely write code to function with the existing messages, but I can guarantee that it will change.