Gday crispy,
Used to be a spray painter on the coast and rustproofed loads of cars and 4x4s with fish oil.Some I did in the early 90s are still going. I guess you could do what you propose buit ive never tried.I have drilled holes and pumped fish oil into places with a spray line that has a 4 way spray on it to coat the inside.You could use a garden sprayer with a long thin tube on the end or something similar if you dont have a compressor.The pro beach fisherman next to my folks coats his old 1980 ute cruiser in every oil he can get his hands on and its still going. There are other products like tectryl and waxoil..... but I use fishoil because I know it works... ive never seen a rusty fish!!
Pete