I have been doing this also. You don't have to go on excursion but I have been present to give the vendor the ticket, the ticket has your name printed on it (the vendor doesn't care if you go, he still gets reimbursed for the ticket), just to be sure.
PS-I believe this only works when booking excursion onboard. If you book online it becomes more messy. They have to cancel and reissue ticket.
Also figure out your latitudes discount in the equation, and even if excursion costs more than $50 it still takes the $50 out of the N/R obc and returns it as refundable