Sorting of a reference selector is automatically done by the selected attribute of the associated object. A workaround would be rendering the reference selector as a form. The sorting can be set in the specific popup form. Anyway, you're probably already familiar with rendering a reference selector as a popup form. Secondly, you can always add a feature request in our Partner Portal.
Normally the attributes are sorted alphabetic. In this case, this was caused by a custom data source. So the question is answered.
This is an old topic, but I also like to have this feature. I have filed a Feature Request for this: FR # 7764.
If I have overlooked this feature in the current version of the Modeler, please let me know!