Most people pin their stock, because it is somewhat removable, in case they need to swap out the stock or something. There is no NJ law stating that it has to be a pin. You just need to prevent it from telescoping. Since there's no straight place to pin that, i would suggest epoxying the mechanism shut.
If you want, figure out which position you want to have it at, remove the stock, place some epoxy in that buffer tube hole and slide the stock on. This will glue the stock pin to the buffer tube. Then, get some epoxy into the mechanism of the stock latch (i know it's a big part on that BCM stock), and this will prevent the adjustment mechanism from being used.