no URI scheme or URN namespace for hashed data, yet (AFAIK).
Actually, there's at least three used with the various file sharing apps:
MAGNET, apparently supported by some Gnutella programs. Uses sha1. Example:
<magnet:?xt=urn:sha1:ZETNTXVNDO2DS32627BQNKIADS3RT3T5&dn=Shareaza1700.exe>
eDonkey ed2k URIs, which use MD4. Example:
<ed2k://|file|Shareaza1700.exe|1330539|b29e2440d84ed468a63971220667786f|/>
sig2dat URIs, used with Kazaa, Grokster, etc. Not sure what hash. Example:
<sig2dat://|File:%20Shareaza1700.exe|Length:%201330539%20Bytes,%201299KB|UUH
ash/kHiGJMFBOMT2rdMU=|>