The simple answer is no.
The two measuring methods possible with App Inventor (and which you already identified) cannot provide what you want. A Proximity sensor measures only very close contact ..a matter of inches (or cm); too small as you indicated. Use of a GPS is possible, depending on the Accuracy of the satellite fix . The 'resolution' of the GPS is never better than two meters using an average cell phone and the satellite constellation the LocationSensor uses to determine its location. Its ability to measure the closeness of two phones (using the locations each phone believes is its best geocoordinate guess) is often on the order of 10 meters as best). Knowing that might be useful. There is no other method one can use using App Inventor. It is not possible for App Inventor to determine if any cell phone gets 'too close' ... App Inventor cannot use information from cell towers to determine which random phones get too close either.
One way is to use your GPS receiver to determine relative closeness of another cell phone, despite the shortcomings, is Social Distancing ... a CloudDB / Location Marker Tutorial . The app will not guarantee identifying separation of the recommended six feet. It can indicate whether a person comes in relative contact of another person that has the same app turned on. Both people have to be actively using the identical app. This is actually very good as it ensures privacy issues involving location cannot be breached. 
App Inventor cannot be used to create a population 'monitoring' app.
A demonstration showing the importance of social distancing (but not ways to determine social separation) is Social Distancing Demo
Regards,
Steve