Oh, that's not hard to do.
One way to do it, or the way I would do it, is to have the rain cannon work by doing a basic spread (defined in the system, not in the bullet patterns) upon death of the first shot. The shots it releases all have another system attached to them (alot of my later bosses such as Vestige use this), and when THEY die, the new system, which also fires off a spread, then activates, firing additional bouncing shots.
Cenrifuge uses an extremely similar idea for it's repeatedly splitting shots. It has a bit of additional code to it, but that was just because I needed specific parts of the pattern going in specific directions, not just in a general spread.