Yes, more or less this seems to be host caching, apart from servents actively sending hostlists through the network which they do not do now.

Encrypting the documents wouldn't be that useful. You could implement authentication instead, although I would not like that for gnutella. Encrypting QueryHits can be used, so anybody passively searching gnutella (i.e. the RIAA) could not tell which files anybody is sharing without actively searching, which is not quite as effective.

Maybe there also would be some legal problems: At least in Germany it's illegal for a detective to ask anybody to do something illegal. And that is what requesting a copyrighted file through gnutella is, no?