Are you putting sea foam in the oil to try and remove the sludge, or are you putting sea foam in the gas tank to try and clean injectors and deposits off of valves?
You’re definitely fine to put it in the gas tank. Most likely you could put it in the oil also to remove sludge if you wanted, and be fine. But it seems to me there would be at least a possibility of a chunk of sludge getting into an oil passage. I doubt the sea foam will be aggressive enough to knock chunks of sludge loose, but you never know for certain so you’ll have to proceed at your own risk. Most likely the sea foam will just dissolve the sludge over time, and you’d have to do several treatments.
I for one am rooting for you and your sludged up, rusty ride. Have you considered cheap synthetic oil changes at an early and often oil change interval (3k miles)? Wal Mart Supertech synthetic is about the same price as conventional oil.