Why would a customer want to have the operation of wing anti-ice with the LED'S deployed as an 'option' ?
Good question
I just looked at the wiring schematics for a number of different airlines, but couldn't find any with the inhibit fitted, but having said that, all of these airlines have manual operation of WAI (apart from the automatic shutoff on the ground). If a disable relay is fitted for an on-ground inhibit, then I'm sure it wouldn't be too difficult to string a few wires from the Flap Control Units to the WAI relays.
I suspect the effect of air shielding was later considered to be not as serious as first thought, so Boeing didn't mandate the fitment of this automation.
Cheers.
NSEU