Rather than "hands on wheel", it should be "eyes looking towards road ahead". My tablet can tell if I'm looking at it. The car should be able to see the driver is in the seat, looking forward, with their eyes open.
Reasonable gaps of a couple seconds should be allowed since humans are supposed to look around but that's just a programming detail.
So say the car realizes the driver hasn't been looking forward for a certain number of seconds, it warns the driver, starts slowing down and attempts to hand control over to the driver.
Really dark sunglasses would be an issue. And sunlight was an issue in the florida crash.
Still, holding your arms up for hours is a recipe for pain.