Two things: I'm certain I read somewhere that as long as you have your NOA1, you are all good to travel outside the US. I'll look for that now.
Secondly, there is a thread on which offices are known to be cool with early walk-ins, though the consensus seems to be that an early biometrics has no impact on the speed of completion of the process.
I'll look for those things now and update this post.
Update 1: That letter extends your status for one year while you're being processed and if you read it, it okays work and travel.
Update 2: Sorry if I'm missing the point and saying things you already know, but you should keep the receipt with your green card as you travel.