Dynamic selects from pre-loaded data is a functionality that was implemented a long time ago (10 years) and it was developed by Dobility, Inc. (as part of SurveyCTO) according to the copyright headers. As far as I remember we have never touched that feature and generally it exists totally outside of the rest of the form spec. We would need to ask @LN but my impression has always been that this is something we don't want to develop.