OK, I have a prototype ready.
I added the check for same parcel, but that only works if the object is owned by the land's owner (or group). If not, it warns you when you rez or reset it.
The user interface script is open, so scripters can modify it to make it prettier or whatever. Datacom details are hidden in two other no-mod scripts (just to help avoid griefing). But the only service the sensor provides is the list of names.
If there's enough interest, I can easily add a feature to the sensor so the only info it returns is the number of avatars (notecard configurable).
There is currently no opt-in or opt-out feature, but that could be added to the sensor, with enough interest. I suspect it's likely. I doubt opt-in would be useful, but I expect opt-out will be wanted.
Post here or IM me ingame to get a kit. Those who requested above will get one now.