While I can't be of any help on the "official rule", I can certainly tell you that I've stored small carryons in that space MANY times. It is even easier to store items under 14A/B, altho I've had FA's tell me that was a no-no as well. My best guess is that it all depends on the FA. Which means that even if the "official" rule is NO, her attitude is really the problem, rather than the specifics of what you were told......When I've been told (nicely) I can't store under 14A/B, I've just said OK, and went about my biz...

Best wishes, Dave