I think the only way to do it is to replace the BIOS keyboard handler.
You should be able to chain the interrupt (look for chain or chain_int or whatever in the help).
Then search the net for Ralph Browns PC Interrupt list to finc which one you need to chain.As I assume CTRL+ALT+DEL is given an interrupt.
Hope it helpsPhil
Found this, not sure how much use it is and the formattings gone to s**t but hopefully it may give you some clues
INT 09 - IRQ1 - KEYBOARD DATA READYNotes: generated when data is received from the keyboard. This is normally a scan code, but may also be an ACK or NAK of a command on AT-class keyboards. may be masked by setting bit 1 on I/O port 21hif the BIOS supports an enhanced (101/102-key) keyboard, it callsINT 15/AH=4Fh after reading the scan code from the keyboard and before further processingthe interrupt handler performs the following actions for certain special keystrokes:Ctrl-Break invoke INT 1B, set flag at 0040h:0071hSysRq invoke INT 15/AH=85hCtrl-Numlock place system in a tight wait loopCtrl-Alt-Del jump to BIOS startup code (either F000h:FFFFh or the destination of the jump at that address)Shift-PrtSc invoke INT 05
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