I would avoid the mash and boil. Too much flavor loss and possible problems with head retention.
I tried 4 oz in a one gallon test batch. Couldn't even detect the coconut. I would say 2 lbs toasted/5 gallons added after primary. That would be my minimum. Work up from there depending on what part of a Mounds bar you like best, coating or filling.
"We are what we pretend to be, so we must be careful about what we pretend to be."