Wino
It would appear to me that the FAA regulations effectively allow a commander to do whatever he wants. I would however point out that the BA does not have an "N" on its tail so is not regulated by the FAA, but the CAA and comply with their regulations (or is it now European regulations through the CAA?).
Furthermore in the UK the CAA FOI’s are designated to each company and all operational notices and policies are copied and approved by the FOI acting in the name of the CAA.
If the FAA don’t like the 747 flying on 3 they shouldn’t have certified it. Meanwhile if they want to criticise the crews decision, produce the risk analysis to back up their view together with the risk analysis of a twin on 1 halfway across the Pacific.