Somewhere (I didn't keep track of where, unfortunately), I read that holding Power and Volume Down for a short period (like a long press) does a screen capture. You'll see a brief shrinking-screen animation to indicate success. Look in Pictures for a folder.
My Android OS is the latest. Have no idea whether this works on all N 7s, but it might. Don't need to be rooted, either.
Have had mine since Dec. 2012, and try to keep up with N 7 info and developments, but just found this. Maybe almost everybody knows about it?