hm more brainstorming.

How about using passive statistics in XPONGS/UPERPONG too? You heared about passive searching, it means sniffing Queryhits on existing gnutella traffic and use the results for own downloads. Passive searching does avoid extra traffic (and provide more hits for automatic research).
So why not using a passive technique to estimate peers in horizon? Every client can collect unique client IDs while sniffing gnutella traffic. On a XPONG it will give the
estimated peers number - together with the summarized files info from TTL1+TTL2 level (files info can not be passively scanned, only the peer IDs).

Collecting unique IDs can be implemented using a binary tree, which stores ID and a time tag from every sniffed descriptor. Every hour the tree should be maintained and reconstructed to delete old IDs (which maybe allready gone offline).