Necromancing this thread, but my post is not sliding well.
Cleaned it , and was wondering about lube, so did a search.
Typical internet foe-rum cacophony.
Brompton sez "No".
Mob says everything, short of drilling for a set screw.
Abrasion for slipping has issues, as do various grits

Would a lube not harm the plastic bushes (AKA liner,sleeve) the seatpost rides in?
Lube seems fine in metal-metal, and carbon, but plastic bushings used in the B are not common and not easy to renew.
The sleeves are not a 'ready to fit' item. They have to be reamed with the Brompton reaming tool and held in place with rubberized superglue.
Though workarounds are applied in the field.