In SM Stirling's A Meeting At Corvallis, the Corvallis Faculty Senate debates endlessly instead of joining the war against the Protectorate so an emergency session of the Popular Assembly authorizes volunteers. Can it do this legally? It just did it.
That is how to do it. Take action while arguing the legality. In Britain a while back, activists in trade union branches knew that if they asked their bureaucratic national union leadership whether individual branches were constitutionally empowered to affiliate to the Anti-Nazi League, an alliance formed to confront the National Front, then the leadership would say no. Solution: don't ask the leadership for permission.