Further, on some Boeing types, WAI don't work if the slats are deployed as it opens the cavity that the WAI air is heating (not sure it that applies to the 777).
I think that applies to the 747, which has no slats, but instead has 'variable camber leading edge devices' that are flat sections which fold out from under the leading edge and are curved by an ingenious 'scissors' mechanism. The resulting void is indeed where the wing anti-ice hot air is directed and so it is not usable with the VCLEDs deployed.
On other Boeings with true slats, WAI is ok when they are deployed as the slats are not flat plates, but hollow, with enough internal space for a piccolo tube.