Hmm... if your ping times are anything like they are in China (often 300+ms) then even in 15 secs you certainly won't be able to determine all active nodes so I do question this idea.

Can you give me some "concrete" examples of just how many nodes this can possibly scale to?

For example - you will never work out all connections if you have connections to 1M+ nodes (and will be consuming vast amounts of memory even trying to) - so has BCNext purposely limited the total network size to something quite paltry like a few thousand nodes?