You can use the Sorceresses' invisibility as a base. Do you want the damage affect as well? If so, you'll have to register if a unit is damaged and if the ability is on. It would probably take, I dunno, about 3 triggers in GUI. Maybe.
Event: an ability is used.
Condition: ability being cast = dummy spell
Event: give windwalk to unit of ability being cast
order unit to WW.
remove WW ability from the unit.
I do not have access to editor at this time, so its kinda badly written, but you get the picture.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.