How can you do this if you couldn't join the league right after the event fired because you were at war?
I ran into this problem in my current playthrough. The religious league war was going on and I was not a part of it because I was myself at war when the leagues were formed.
What I did was to declare war a no CB war on the Catholic league leader (for some reason, Tyrol was the emperor in my game) and proceeded to occupy half the HRE (including Austria) + Spain. The other half was occupied by the Protestant league.
I marked as a special interests all the Catholic league members and waited for them to separate peace the Protestant league leader (Brandenburg) before peacing them out myself.
I had to stay on "call for peace" for about five years before the Catholic finally capitulated to the Protestant league, which was the outcome I wanted to see.
So, my advice would be to get a parallel war going with the league leader you want to crush.