You can imagine it, can't you? You go to buy your ticket online and you're given the option:
PRIORITY ON-BOARD PEE - £1.
(If you do not avail of this option, you will be faced with a £5 charge in the event that you cannot keep your legs crossed until we land.)
Oh yes, I can see it now. Two lines in the aisle. One line for people waving their priority peeing vouchers, the other weighing up the option of flushing £5 down the loo or peeing in their pants, while vowing next time to pay up-front just in case.
We fly SAS to Dublin anyway, as they give us a 100 kg luggage (i.e. shoe) allowance with our tickets so it works out cheaper, even without paying to pee. With my tiny bladder, it would be a no-brainer.