Not trying to drop a turd in your punchbowl and I certainly am no lawyer. But I was reading quite a bit earlier today about a 'tool' that used a magnet to basically turn a BB into an oversized extended mag release and the concensus was that the product would make 95%+ Bullet button equipped CA legal rifles illegal due to fact that it was basically disabling the point of the bullet button by turning it into an extended larger magazine release button that did not require a tool to use and the fact that it could easily be left on the rifle made it a part of the rifle not a tool.
Anyhow, obviously different situation other than the fact that the tool your offering is attached/stored on the gun. The fact that you have to use a separate magazine to actually release the BB probably has you in the clear but you might want to talk with some folks around here that are knowledgable in this area. I know it is a matter of opinion, but Gene seems to lean toward not having tool attached to your rifle. Just my two cents. I think it's an awesome idea and wish you the best. Just wanted to make sure it's all good and not felonious.