I´ve asked people in the self-service-restaurant on the NCL SUN, if they have a pool for the staff. But they negated it.
I asked crew members on a RCCL ship and they told me the same.
When you look, which waiters are working in the selfservice, you can see, that they change from time to time. So some can participate from the tip at the end of the cruise, because they are also serving in the dining room and some are not.
Maybe on other cruiselines they have created a pool. I know that on the German AIDA ships, where no tip is required, they place a big glas at the end of the cruise for tips from the passengers.