My RT-11 boot disk with DECUS C worked a treat on the real machine. It took 16 seconds for it to draw the mandelbrot set in ASCII art on the actual uPDP-11/53. Here is a picture of the real thing in action:
I would say that's a result. My next project is to use memory-mapped I/O in C to communicate over the second serial line. This may allow me to hook the PDP to my serial-to-wifi board.