In the "view title" family, use a yes/no parameter on the visibility of the standard scale that reads off the drawing. Do this by clicking on the scale label, go to element properties and click the small gray box to the right of the "visibility" row. Create a parameter name for this and it will autmatically be set to "Yes/No." This will create a check box that will allow the parameter to be turned on and off. Next create a text field with the same idea that simply reads "Scale: NTS." Make sure this has it's own "Yes/No" parameter. Now set up Family types that utilize this idea. When the family is loaded into a project, the viewport "style" must be set to use the appropriate "family type" from the family just created. Therefore, the viewports will have to different "types" (styles), one called Viewport - Normal, and one called "Viewport - NTS" (or whatever names you like).