Can you use the scanner software from another application (or as a standalone application)? That could allow you to fill in the data in a blank Photoshop document while it's scanning, then you could overlay the scan into the document later.
I'm wondering if Photoshop version is going to make a difference from the point of view of image size. Have we always been able to paste a layer bigger than the canvas for instance? I am pretty sure the answer is yes.
But Noel's idea would work perfectly, and I suspect the OP's version of Photoshop is too new to support TWAIN.
So there you are. Create a new document and start filling in the EXIF data in Bridge. Then duplicate the scanned artwork into that document and flatten.
Good point Trevor... Placing the artwork onto a minimal document Photoshop, then doing Image - Reveal All (and following with Layer - Flatten as you said) should do the job of sizing the canvas.