I think the solution to this problem should be more human than metric. I have a human solution to it. It might not click with everyone. tl;dr - be around as much as you can, it shouldn't be that one guy is putting lots of extra hours all alone all the time.

I feel that folks who are not able to spend extra time due to family/life's other commitments should (few times a week) just be around the person who is actually spending extra time. So basically, they may or may not really work on their stuff but being around to share ideas, celebrating small milestones matter a lot. This also shows they care and ultimately thats what matters.