I don't know how to set something up like that, although it's certainly possible. If it's not possible it would be pretty easy to have two sheet formats, with the note linked to the custom property in one and to the configuration name in the other, and use whichever would be appropriate in your Drawing.
You need to create one more property and then use design table and excel intelligence to populate this new property with either drawing number or configuration name. And then link this property into drawing.
The only way that I can think to get what you want is to have the Drawing Number field populated with $PRP:"SW-Configuration Name" by default inside the Property Tab Builder.
You could do it 1 of 2 ways (as I see it):
1) Create the Drawing number as a textbox and put $PRP:"SW-Configuration Name" into it as the default value.
Pro 1: If the user doesn't do anything with it, then the Configuration Name will be Drawing Number automatically.
Con 1: The Property Tab Builder interface never shows the Evaluated value....so the user will see $PRP:"SW-Configuration Name" in the text box.....which could confuse some users.
Con 2: Also, if the user accidentally deletes the $PRP:"SW-Configuration Name", then they would need to know the syntax to get it back.
2) Have Drawing Number as a drop down menu with $PRP:"SW-Configuration Name" as one of the choices, and allow custom values to be added.
Pro 1: The user can't accidentally delete $PRP:"SW-Configuration Name" since it is one of the choices in the drop down menu.
Pro 2: Also, the user can add a custom value making it the proper drawing number.
Con 1: There is not a way to have a selection in the drop down menu as a default selection.....so if the user does nothing with it, it will remain blank. They will have to choose to add a drawing number or the configuration name selection.