This is my answer to a super simple question that had many scattered partial answers around the internets. Kind of a gist.
I am no longer working with xpages, but at home I still have my TDL app running. There is a very simple to do list view I compiled from some example template. The "mark done" button has been sitting as a dummy for years now - never found the easy way to get that function happening.
After googling my way to some Stack-O and Developerworks and a few Domino blogger q-and-a threads, I put this little pile of SSJS in place and ta-da! No original work here, just the gluing and duct taping - see answer: