Glad you mentioned that and even happier you were able to make it work. If they get free of your hooks when you have the seatbelt, they are WIDE open for the N/S choke. I do this a lot too. Just so everyone knows, they free the hooks, you get to your knees behind them while keeping the seatbelt tight, crunching their head down, and you sprawl with the seatbelt. After they've lost their balance, and the seatbelt is the only thing keeping their shoulders and head off the mat, let your right hand go so their head falls and immediately wrap up the head and bam, they've fallen right into the N-S choke!
It also works in reverse. If you have a N-S choke and you burn your arms out, give them a little room to sit up. You can even bring your knees up under their shoulders so they're in your lap. Lock up the seatbelt, walk 'em forward enough to drop to your side with hooks in. Often you can go straight into the RNC before falling to your side.
There is a pretty cool relationship between the RNC and the N-S choke that is rarely talked about. I dunno how well they work in competition, but in class it works well both ways.