You can get a lot of bombs in arena settings but not unlimited.
You're correct. It's only guns and cannons that have the multiplier. There's no multiplier setting for bombs, however, if you use the air launch option and exit flight after each pass you get "unlimited" bombs.
Air launch is built into the map. In the TA map use the bombing area bases, 20 - 23, and use different runways for different spawns.