I feel that there is room for a proc like that, but I can't say how powerful it should to be. Probably just a free Arcane Barrage won't be enough to make it more relevant, don't know. But why not multistrike? Even if mastery is our main stat, Arcane Barrage multistrikes look awesome. But I'm afraid we are going too far away
as you say that would need to retune the rest of the spec.
Agree the idea still feels random, and even too similar to Arcane Missiles. Even if often we would keep the proc to use it on the right moment (4 charges, need to move, several enemies, arcane power/prismatic crystal), many times it would be just cast Barrage when it procs. That's not necessarily bad considering that we spam Arcane Blast a lot and Arcane Barrage can proc Arcane Missiles. But it could be more fun or less random.
Maybe it could be related to other spells like Presence of Mind, instead of being a random proc.