I had a job hanging doors in a new house where all the doors and jambs were delivered premortised. All I had to do after hanging the jambs was to screw on the hinges, stand up the doors, and then mount them in their jambs. After a couple of days of straddling doors with my legs while affixing the hinges, it occurred to me that I could simply stick the end of a door into a corner, as shown in the drawing above, and the door would be held upright.
This setup, if you can call it that, is stable enough for routing mortises. For more stability, you could brace the outboard end of the door with a pipe clamp at the base. In finished rooms, protect the inside corner of the wall with some sort of padding.
David Kalin, Kaneohe, HI