Synchronous postMessage for Workers?

Jonas and I were having an offline discussing regarding the synchronous
Indexed Database API and noting how clean and straightforward it will allow
Worker scripts to be. One general Worker issue we noted - independent of
IDB - was that there are cases where Worker scripts may need to fetch data
from the Window. This can be done today using bidirectional postMessage,
but of course this requires the Worker to then be coded in now common
asynchronous JavaScript fashion, with either a tangled mess of callbacks or
some sort of Promises/Futures library, which removes some of the benefits
of introducing sync APIs to Workers in the first place.

Wouldn't it be lovely if the Worker script could simply make a synchronous
call to fetch data from the Window?

GTNW.prototype.end = function () {
    var result = self.sendMessage({action: "prompt_user", prompt: "How
about a nice game of chess?"});
    if (result) { chess_game.begin(); }
}

The requirement would be that the Window side is asynchronous (of course).
Continuing the silly example above, the Window script responds to the
message by fetching some new HTML UI via async XHR, adding it to the DOM,
and only after user input and validation events is a response sent back to
the Worker, which proceeds merrily on its way.

I don't have a specific API suggestion in mind. On the Worker side it
should take the form of a single blocking call taking the data to be passed
and possibly a timeout, and allowing a return value (on
timeout return undefined or throw?). On the Window side it could be a new
event on Worker which delivers a "Promise" type object which the Window
script can later fulfill (or break). Behavior on multiple event listeners
would need to be defined (all get the same "Promise", first "fulfill" wins,
others throw?).