Can you put a new Text Field on top of the area where the GrandTotal would appear? Kinda like putting a transparent film over the top of a picture and writing on that? (Or a new layer on top of an existing picture.)
That's how we handle text field "hints" on our forms. There's a Text Field, and on top of that I put a Text Box. When the user enters that field, I make the box hidden.
Would something like that work?