Have you tried de-ionised or distilled water? I'd imagine if you flushed your system fully of contaminants and then bled with that stuff your seals would keep for a fair bit longer than if you used tap water.
Oh, and for anyone thinking of it, mercury won't freeze until it drops to -38.72c, so solid brakes won't be something you need to worry about in this country (unless the freak weather continues )