The DNS system makes logs (and metrics) for how many DNS queries it gets from each country. Obviously that doesn’t quite correspond to NTP queries, but it should be a good proxy.
My plan is to calculate for each country with less than X servers (or X “net speed capacity”) how far off it is the median/average/some reasonable comparison country and then supplement with servers either from the global zone or that have explicitly joined some other “extra global” zone. (If we use the global zone I’ll add an option to remove a server from the global zone).