To do this you do not need difficult PeopleCode, but you do need to do some funky stuff in App Designer.
First off you need somewhere to put the name of the prompt tbale that you wish to use. Therefore on you page palce a field for the DERIVED record with a name like EDITTABLExx (where xx can be nothing or a number, which on you use doesn't really matter as long as it is unique in the component). Make this field invisible and display only.
Next you need set the Edits on the Field that will trigger the prompt. It needs to ba a Table Edit of type Prompt Table Edit, and the prompt table is the name of the field that you used from DERIVED, with a % in front of it (ie %EDITTABLE12).
Now comes the PeopleCode bit. All you need to do is set the value of DERIVED.EDITTABLExx to be the table that you wish to use (DERIVED.EDITTABLE12 = Record.BUYER_ACTIVE_VW). You will need to set an intial value vale for in in RowInit somewhere, and then you will have to put some FieldChange code to update the value of DERIVED.EDITTABLE dymanically.
Let me know if this helps, or if I can clarify any of this.