I've hiked Isle Royale from end to end twice: once via the Greenstone Ridge Trail and once via the Minong Ridge Trail. Unfortunatley, I don't know of a way to do a mail drop w/perishable items.
During the summer months, there is a small store open at both Rock Harbor and Windigo. You can purchase stove fuel at both locations as well as lunch/snack type foods (nuts, cliff bars, candy bars, etc.). Assuming you are planning to hike generally from one end to the other and back again, you could pack breakfast/dinner foods for the complete trip, but pick up fuel and snack foods for the 2nd half of the trip at one of the stores. This would be expensive since the store pricing is similiar to airports, but it would reduce some weight and bulk at the outset. I wouldn't recommend trying to find breakfast/dinner foods at the store.