I bought some copper ones from Walmart and just to test I put a few in water in a ziplock bag and the BB’s promptly rusted badly.
They would work in a dry climate and maybe even a cool slightly damp climate but not in a warm humid one. When I lived in the Seattle area I had a tin collection that never rusted. I moved to Tennessee and they began to rust inside - not a lot but some. I should have put silica or at least a paper towel inside each tin and changed it out each year.
I decided to not use the copper BB’s in this climate and I wouldn’t want to sell one with it - who know where they may move. They may have to put the doll in unheated/uncooled storage and come back to a sad state.
If you put it in a balloon I would dry the BB’s first. Heat your oven to 200 degree. TURN IT OFF! Put the BB’s in a large glass or metal pan - one layer deep. Leave for an hour. Take out and cool completely and then put in the balloon. I would leave it in there for a month and then open to see how the BB’s look before I would decide it is safe. If you open on a humid day I would dry the BB’s again.
I’ve never dried BB’s but I do this with popcorn, stale crackers, chips etc. and it works. The popcorn is more crunchy than it is right after I pop it.