I have not yet heard anyone discuss where they believe they have seen R2 yet.
For what it's worth, I am fairly certain that I know where he was placed.
On my third viewing, I deliberately searched the background for him, because I was familiar enough with the story to not need to focus on the foreground. I spotted him near the end of the film, in the scene where young Spock meets Spock Prime. Near the end of their conversation, with about 10 seconds left, there is a camera angle of Spock Prime where a shuttle can be seen in the background. R2D2 is standing to the right (our right) of the shuttle. I made out his dome, legs, and his repair tool/welder conducting some repairs to the side of the shuttle.
Guess we'll see. My friend who was with me thought I was a big nerd.