I am thinking a little clearer right now. Let me explain a little better. I don't think pong cacheing would be the right correct term, so I don't know of anyone that does this. However, what LW is doing with 1.9 is this:
A client (branch, or 'leaf') connects to an ultrapeer and uploads a filelist (it's not really a file list, just a 'representation' - ask afisk for info if you want to know more about it). The actual searches are perfomed by ultrapeers. Search queries are compared to the file lists of the branches, and if any seem to match, the search is passed along to the corresponding user. So, the search will only reach the branch client if it is a relevant search. You can see this in action in 1.9 beta. The Monitor tab shows incoming searches, and all of the searches have words or phrases matching files in your library.
Multiple identical queries are merely directed to the same clients that previous identical queries were directed to.