I've had a similar problem with parent who hasMany children. I solved it by using grid cellediting plugin, which works great for editing parent data, and for associated children I created a custom editor which extends Ext.form.field.Picker and handles get/set manually. And I also use a custom template to render children in grid. In your case you could extend Ext.form.field.Combobox which also extends Ext.form.field.Picker and override get/set functions. But for that you'll need to get your hands dirty and dig into the source code. Hint:

Ext.util.Observable.capture(observable, function() {
    console.log('capture', arguments);
Anyway, you'll have to handle this manually. But it would be cool if you could handle association editing through config.