Personally, I would want to keep anything oily or waxy (including cosmoline) far away from my brake linings. I want friction, not slippage between drum and lining.
I see no problem with protecting the drum for storage, provided there is no way for the protectant to get on the lining and that it is all cleaned off before trying to use the brakes.
For longer term storage while the truck is under restoration, why not take out the shoes and linings and protect the drums, then clean and reinstal the friction parts before putting it together to go on the road. This way you are going to do an inspection shortly before driving, make sure all the mouse nests are out of the drum etc. Also, the linings can't freeze to the drums to make it hard for you to move the truck during restoration. If doing this, you might want to secure the wheel cylinders to keep them assembled rather than having them expand and possibly pop the pistons during storage.
I would also want to clean any preservatives from the outside of the drum before driving because I'm not sure if they would resist the heat generated during braking without flowing into places they are not wanted and either stinking, smoking or burning when the brakes are used.
