I think that this could be done safely and within the rules as long as your device is well enclosed for protection and as long as a human will never need to be in contact with the surgical tubing when it is cocked (stretched out).
Check out these
semi-auto rubber band guns that have a rotating mechanism that releases one band at a time. In my opinion, you could make something like this to release at least a few balls in rapid succession. Re-setting the device would be a challenge, but again, I think that it can be done.