This isn`t being negative or pro BuMW but is a realistic view.
Sergeant B has it about right.
We do a lot of warranty insurance for automotive manufacturers.
Few - if any - manufacturers will extend any kind of warranty payment to a vehicle that has been written off. The legal impications are simply too onerous. Of course you can find warranty cover for second hand vehicles of all sorts, but that is not the same.
The best you can hope for is a 'Good will' payment from BuMW (or much more likely the dealer) to perhaps match in with the dealer's suggestion to bring the vehicle back after the winter.
Even that may be clutching at straws. BuMW themselves may well claim that they (the manufacturer and 'owner' of the manufacturer's original warranty) would have been more than happy to have repaired the corrosion when you first brought the bike in. It was the dealer who suggested that the work was held over, not BuMW, was it not? Quite why the dealer did suggest it - and you accept it - we will probably never know, exactly.
Anyway, what are we talking about? An hour or two with the paint remover gloop and scraper tools, followed by primer and suitable paint. Job done. Cost? A few pounds, surely?