Custom event hacked up to support defaultFn that can return a promise to delay after subscribers. POC, mostly untested, so YMMV.

gistfile1.js

/*global SM:true*/
'use strict';

// Publish a very special, promise-compatible event that supports the default
// function behavior to chain on internally created promises before executing
// the after() subs. on() subs can still e.preventDefault(), which will
// reject the promise and thus skip the default behavior and after() subs.
var asyncEvent = eventTarget.publish('foo');

asyncEvent._firing = new Y.Promise(function (resolve) { resolve(); });

asyncEvent.fire = function (data) {
    var event = this,
        args  = Y.Array(arguments, 0, true);

    event._firing = event._firing.then(function () {
        
        event.details = args;
        
        // Execute on() subscribers
        var subs = event._subscribers,
            e    = event._createFacade(data),
            i, len;

        if (subs) {
            for (i = 0, len = subs.length; i < len; ++i) {
                // TODO: try/catch?
                subs[i].fn.call(subs[i].context, e);
            }
        }

        // Resolve the _firing promise with either false if it was prevented, or with a promise for
        // the result of the defaultFn followed by the execution of the after subs.
        return e.prevented ?
            false : // Doesn't support preventedFn, but could
            Y.when(event.defaultFn.call(event.context || event, e)).then(function (e) {
                // Execute after() subscribers
                subs = event._afters;

                if (subs) {
                    for (i = 0, len = subs.length; i < len; ++i) {
                        subs[i].fn.call(subs[i].context, e);
                    }
                }
            // Catch errors/preventions and reset the promise state to fulfilled for
            // the next call to fire();
            }).then(null, function (err) {
                Y.log("Error in defaultFn or after subscriber: " + (err && (err.message || err)), 'error');
                return false;
            });
    });
};

asyncEvent._fire = function (args) {
    return this.fire(args[0]);
};

Luke, need some more help:

The event cannot be targetted to another EventTarget instance.

var someTarget = new Y.EventTarget();
myobject.addTarget(myTarget);
myTarget.on('*:foo', function() {
    ... <-- nothing happens
});
myobject.fire('foo');

Luke, thanks!

I got the final code working: created Y.Model.prototype.publishAsync
https://gist.github.com/ItsAsbreuk/6712217

One small bugfix:

line 40-42 should be arrounded by:

if (subs) {
}