It is nearly impossible to really keep the upper back arched when you are in a snatch grip position.
With the snatch, and OLY lifter will not be positioned like a deadlift at all anyway...so you can't do a "deadlift" from the snatch position really. In a snatch you will start with your shoulders in front of the bar (deadlift would be from behind the bar) which would allow you to have a flat/arched upper back. Shoulders behind the bar changes that with that grip.
You aren't going to be pulling any crazy weights in that position anyway...so I wouldn't get overly concerned.