Hi James,
With the release of ArcGIS Online yesterday, add some support for subtypes. In BAE, when you click the drop down, you will see the sub type descriptions. Once you select one, then the textbox will keep the code value. Not the full solution but atleast a solution that shows the value. the underying reason is the way dojo combo box works (or lack thereof).
Also, it takes the type code from the rest endpoint, which only support one. so if you have multiple subtypes only one will show. Give it a try.
Thanks,
Previn