Well… the SOP came straight from the airline.
Yes, I’d expect there to be flexibility. Those with special needs or infirm should be able to use the closest WC. Other pax should know the drill… go before or after the carts have been in the aisle, or select a seat at the rear beside the WCs?
It seems to be an issue of pax entitlement. When there’s a barrier, i’ve seen zero main cabin pax come forward. Without the barrier, they sometimes come forward despite the announcement their WC is located at the rear.