I am not aware of further rulings on this. Yes, I do agree that for taxes that are proportionally affected according to the price of the ticket (such as the Mexican VAT), it is reasonable to expect a proportional refund.
This is certainly not the case for US security fees or UK passenger duty taxes (except when going from F/J to Y), which are fixed.
I think AA entirely controls the 'carrier surcharges', so a proportional refund there is more than reasonable. Otherwise, they could charge 1 GBP for the actual ticket, 4999 GBP on carrier surcharges, and avoid any meaningful responsibility. Hopefully I'm not giving them ideas!!!