The system reset is interpreted and generated by the system gate array (Gary), the CIA only handles the serial communcations with the keyboard and cannot physically generate a system reset.
Reset is sent from the keyboard by generating a long pulse (I forget the exact timing, probably 100ms or so) on the clock line. This is detected by Gary, which then generates a logic low on _KBRST, therefore causing a system reset. That's obviously why _KBCLK appears at Gary, but not _KBDAT, as the clock line is otherwise completely useless by itself.
By the way, the _ in front of a signal name means that it's active low, in case you didn't already know.
The system software should have no affect on the reset operation. If reset control had to be actioned by the CPU, but the CPU was in a frozen state, how would you ever be able to reset the computer??? Think about it.
So to answer the question of disabling keyboard reset:
This could probably be done by lifting the _KBRST output pin from Gary (pin 36). There's an external pullup on this line, so it would correctly reside in a logic high state.
Not sure if Gary _KBRST is used for anything else, so it might be better to disconnect the _KBCLK input instead (pin 44). Note that you'll probably need a pullup resistor to 5V on this disconnected pin if you do this.
And how do you "accidentally" press the reset key combination anyway? I could understand if your hand was 12" wide and you typed like a hyperactive gorilla. But most people aren't quite that badly mutated...
