Thereby extending one's discomfort by a factor of days rather than hours.

IB fly MAD-LAX 3 days a week. The concept of IB Y for 13 hours followed by TBIT and a LAX transfer in December is right up there with the Amtrak option, in my view.

You might think about AA's 777 to LAX, and as a BA elite pick the exit row (31) for great legroom. Then either hire a car and drive up 101 to SF (pleasant and reasonably fast in December) or else go over to the dark side and try VX between LAX and SFO - possibly the best domestic Y (and definitely the best domestic F) in the US.