OK, fair warning. When faced with an unusual problem my mind frequently defaults to a solution using cardboard and gaffer’s tape. Question: Is the rear camera usable when in the POS stand?
If so, get a piece of dark gray/black posterboard large enough to leave a 2 - 3 inch margin on each side of the document to be scanned, and attach a small clip to hold the paper in place. Open ScannerPro, Press the + button, hold the board/document in view of the rear camera and proceed normally. Done.
If the rear camera is unavailable, wait until one of the more intelligent members of this forum responds to your question.