How nasty of BA. If they have a representative at NRT before the AY flight departs, might be worth a try talking to them. If that does not work, show all your paperwork of the original ticket to the dragon at the lounge entrance and explain that you are on an award ticket that the airline needed to amend and that award tickets are eligible for lounge access. May or may not work.

The Plaza Premium at HEL accepts walk-ins against a cash charge. Would probably be quite difficult to get BA to reimburse afterwards, though.