Last time, the shows went to air without the writers. IIRC the way it worked was that they did away with sketches and monologues so there was no writing, basically just interviews and improv. Leno did write his own monologue at one point, despite being a WGA member and deservedly caught shit for it.
Letterman and Conan paid for the writers salary out of their own pocket for the duration of the strike.
SNL and Colbert set up live shows to raise money for the striking writers.
All of this was done in solidarity with the writers, which were never replaced or supplemented throughout the entire 08-09 strike.