I don't think it is possible, because in pro program you can only modify a part or an assembly through parameter and you can't force the system parameter rel_model_name() to be equal to a user parameter !!!!
If you explain better what you wish to do, perhaps I can help you
Dou you simply assembly the workpiece or through pro/program pass to it some values or parameters? If you canghe via program an assembled part then you can instantiate it, that is exactly a save_ascommand. If ins't a problem for you have a family table of wokpieces this could be a solution.