Generally, if you select 'Quick Boot' (or whatever else it might be called), bypass some of the POST checks, in the BIOS, the PC won't check the keyboard except for watching for stuck keys. I just tried it on my new-ish Asus Mobo with a 945 chipset.
Quick Boot disabled - No keyboard (USB or PS/2) - Failed, wouldn't continue booting.
Quick Boot enabled - No keyboard - Passed.
Quick Boot enabled - Keyboard, holding down space bar - Failed, but continued booting.
EDIT: There's usually a couple of settings for 'Halt on Disk/Key errors' that can be turned on/off.
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