i am not aware of any servent that supports that at the current. i think i have read that bearshare searches the meta-tags of mp3's also but havent heard about searching text documents. also i think in gnotella there is an option "return meta-data" or so. but i guess this relates to mp3's also.

but this would be a good feature-request. although i do not use gnutella for documents, enhanced searching by meta-data (and showing the results, e.g. kbp/s for mp3) would really nice ...