After many many tests, I've discovered that the first thing to test is whether the command or script concerned really works (e.g. using command line) or not, and then the second thing to test is whether the custom key combination chosen (e.g. Ctrl+Alt+D) conflicts with another one in use.
Unfortunately, the second problem seems much more difficult to resolve because the current gnome-control-center system apparently does/can not warn all such conflicts.
For instance, in the case mentioned here (
gnome-control-center display) a custom keyboard shortcut such as Ctrl+Alt+D doesn't work - without any warning when assigning it - but a custom keyboard shortcut such as Ctrl+Super+Backspace does work.
Therefore, one might need a lot of trial-and-error work to find a good keyboard shortcut.
It might also be necessary to log out and then log back in for the newly assigned keyboard shortcut to work.
In addition to many keyboard shortcuts used in various application, here are some Unity keyboard shortcuts to avoid: What are Unity's keyboard and mouse shortcuts?