If you wanted to hijack Ctrl/Alt/Del, you could modify /etc/inittab (next-to-last-line). I don't know if you can add other key combinations there or if Ctrl/Alt/Del is special (perhaps because of the NMI?).
Some of us who still have to use Windows get all irritated when we walk up to the key board and, out of habit, give it the 3-fingered salute (to log in or return from screen saver) and the system reboots. Still, there's probably some good reason to leave Ctrl/Alt/Del the way it is.