I did run the test in DNG mode. I believe that I did not run a long enough exposure time since it was only about 2 seconds however the "busy" message did persist for about 2 seconds after. I will rerun the tests with a much longer time to confirm that this feature is working.
What you are trying to confirm is that CHDK can turn Dark Frame Subtraction
OFF on your camera during long exposures. The Canon firmware has it
ON by default for those long exposures.
So if you set the CHDK Dark Frame Subtraction menu item to
AUTO or
ON with a 15 second exposure, you should see the complete shot take 30 seconds ( 15 for the shot and 15 for the dark frame ). If you set it to
OFF in the CHDK menu, the same shot should only take 15 seconds to complete.
The test is to confirm that CHDK actually tricks the camera into not taking the extra dark frame with every long exposure.