How can physically disabled people who aren't flying first or business class use the lavatory on a Lufthansa A340-600?

Lufthansa uses the Airbus A340-600 on a large portion of its long-haul routes. According to the seatmaps for the two different configurations it uses, all of the lavatories for premium economy and economy are accessed via a staircase into the lower deck (underbelly) of the aircraft, in both configurations. The only toilets on the main deck are located either in front of the plane in first class, or smack in the middle of business class.

This presents a problem for physically disabled passengers (specifically those unable to climb stairs) who aren't flying first or business class.

How does Lufthansa accommodate such passengers, when they want to use the lavatory? Are they offered access to the premium cabins' toilets? Or do they have no choice other than to go down the stairs?

Ideally, I'd prefer an answer from someone who's either flown on this plane as a passenger and has first-hand experience of this issue, or is familiar with the internal Lufthansa policies that govern this.