The responder can still respond from imagination, but at least now both pilots (as opposed to just one) would have to slack off to let a problem slip through... so at least it's some improvement.
My operator has tried to fix this by requiring a tactile (or pointed) movement along with the response for those critical items in a checklist completed as challenge/response (I forgot to mention this in my post above).
For example, during the after takeoff checklist, we are required to look at and touch the bleed air switches to make sure they're on, and then touch the pressurization switch to make sure it's in auto. I swear they did everything but have a wooden spoon in the sim when they added that to the SOP and I still feel as though I'm being watched when I fly the line.