Seems like they could address this by just changing procedures.
If one pilot wants to take a nap, require a third crew member to come sit in the cockpit and make sure the other pilot doesn't doze off. When two pilots are awake, they can monitor each other, but if one is (intentionally) asleep, they can't do that, so have someone take over that function.
The two would have to fall asleep at the same time though. Otherwise, as soon as the SIC falls asleep, the third one would trigger some special protocol (like a kick in the tibia)
Seems like they could address this by just changing procedures.
If one pilot wants to take a nap, require a third crew member to come sit in the cockpit and make sure the other pilot doesn't doze off. When two pilots are awake, they can monitor each other, but if one is (intentionally) asleep, they can't do that, so have someone take over that function.