The closest I have come to doing what you are trying to do is making some modifications to the community solution found here,
https://www.aras.com/projects/project-view.aspx?id=2FF460BABB6B4092B2938E718E6DA4A8.
There is a method named Char Value Dropdown List that is hooked to the onEditStart event of a property on a relationship itemtype (much like part bom). The method dynamically changes a cell on a grid from a text field to a dropdown list (you probably don't need that functionality) and populates the dropdown list with labels and values.
Hope it helps.
Dennis
