This is something which has been done by bus and coach operators for many years and in the last ten years on the railways when guards have decided to strike. Might be a first for the airline industry.
Certainly ContinenatlClub is right about recruting specifically to cover for striking employees. The agency issue is a very grey area, but there is no problem with, for example BA using VS or BD crew providing they are paid by the employing company. They could also get a charter company like Titan that BA used to use when they had an aircraft shortage in the days before BA Connect.