[_ck_]

Core hacks would not be needed for this, just special views and filters. The same way my hidden-forums plugin works, you could make topics available or hidden based on location.

Do you really want to track at the POST level or rather the TOPIC level which would create far less data to churn through?

There is already someone using bbPress for a travel agency of sorts with dozens of destinations all linked together. Each city is like a forum in itself. I remember it well because it was very difficult to filter out the dozens of pseudo-forums from the top1000 list.