OpenNapのプロトコル仕様書を見てみたんだけど、

215 (0xd7) request resume [CLIENT]

<checksum> <filesize>

client is requesting a list of all users which have the file with
the characteristics. the server responds with a list of 216 messages
for each match, followed by a 217 message to terminate the list

これ使えませんかね？
ファイルサイズを指定して検索できそうだけど。