thats a cool idea, but i agree that a hand lever would not have anywhere near enough travel to actuate the coaster brake. i made the suicide brake bike, and there is alot of distance covered when i pull back on that handle to brake. your on the right train of thought though, thinking out of the box is a great start.
since you only have one brake boss how bout this, mount the brak arm and hand lever and instead of a brake pad, mount a hand with a certain finger extended, that way when you pull the brake lever you can "waive" to the cars that arent giving you your lane.
maybe use the brake boss to mount a light.
maybe use it to mount a really long antena w/ an orange flag on the end like the big wheels when we were kids.
i look at every hole in any part of any bicycle as a place to mount something.
fast eddie outty