As far as Rules As Written go, I'd say that you automatically hit and score a critical hit, dealing some multiple of your normal non-lethal damage... with a Fort Save-or-Die.The DC for the Fort Save would either be 10 (if you consider the "damage dealt" to be zero, since no hit points are actually subtracted from their total), or 10 + non-lethal damage dealt (since it's still 'damage').
The suggestions of Save-or-Unconscious make for a fine house rule, but I quite like the chance of accidentally causing a brain hemorrhage...
-Hyp.
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