Nice idea, but some difficulties:

- The detection distance for typical tags is only a couple of centimetres at most. You might be able to put a tag onto the other side of pocket lining or something like that, but hardly to different pockets

- You cannot control from applications when or how to poll for tags, the system keeps that to itself. N9 only polls for tags when the phone is not idle (when the display is on). So checking every 5 seconds on an idle device is out of the question.

While you could bring a tag to the RF field while the display is on and keep it there after device goes idle (NFC remains active until tag leaves the RF field), that is quite expensive batterywise.