I agree that many hours of work would be needed.
You could argue that the TransDEM user could just create a route for the requestor consisting of a huge chunk of terrain, then email the .cdp file and let the requestor delete what they don't want. Of course, this would result in a route too big to save as a .cdp file, not to mention too big to email. So, there would have to be a lot of back-and-forth communication between them. Now finally, they agree on the needed terrain and the .cdp file is emailed. But now the requestor emails the TransDEM user and says "I was wondering if you could include the X mountain range as background - could you re-do it?"
You can see where this is going.