Have similar things not been done in the past? I remember once seeing
a tube map which had one branch of the Northern Line marked as one way.

Wasn't the entire 'City' branch of the Northern closed for major works
back in the early-mid nineties? IIRC this was also the time when they
eliminated the 'dangerous' island platform at Angel, built a new
tunnel, slewed the track, and provided separate NB / SB platforms?

M