It must be one of the multi-protocol clients, probably Shareaza or maybe Morpheus etc. It's not a standardized or documented feature.

In theory, a HTTP GET request for

/uri-res/N2R?urn:ed2khash:...

should work. I think you only see such queries if you don't implement the Query Routing Protocol (QRP). At least, it wouldn't be a good idea to pollute the query routing table with those ed2k hashes. For such purposes a DHT should be used. LimeWire, for example, does not route urn:sha1 queries any longer.

Gnutella will sooner or later be extended with a DHT for such purposes.