If you use a design table, you might get away with it. Using the LEFT and RIGTH text functions inside Excel (and some others). Then have excel populate a custom property with the result. You would have to do that at the part/assembly level.
Then use the custom property for your part number field on the drawing.
I think that you can even create the design table into your template so that it will auto populate.
Thanks, I will look into a design table.