Read the law mike. Theres nothing in it that says it has to be permanent for the mag catch. Technically, by vebatim, it renders the rifle not an assault rifle and herfore registration need not apply. There are tons of gun stores selling ar15s again just like this, with either an mr2 in place or a locked mag. Even got ny2a lawyers backing it up, so yeah, good to go.
Now doesnt mean you wont get arrested and have to fight it in court but youd win regardless despite having to pay for a lawyer based on how the law is written