When performing a secondary fermentation, it's recommended to use a glass carboy rather than a plastic bucket, with one of the main reasons being that it reduces head space so that less oxygen is sealed up with the beer. If that's the goal, could you instead line a sanitized bucket with a sanitized food-grade plastic bag, siphon the beer into the bag, then seal the bag so that there isn't any head space at all?
BYO mentions lining a bucket with a plastic bag to help with sanitation, and assuming the bag is BPA-free, food grade, etc - would this be safe or at all effective?