Normal http-downloads don't use hash-strings, so it wouldn't work that easy.
BUT: If the owner of the webpage thought about his or her slow connection, and used magnet-links with the http-url as alternate location, you can already use Gnutella that way (though, afaik, phex doesn't jet support magnets, or does it?).
Check this magnet-link:
magnet:?xt=urn:sha1:IJUP7VDGLRBMVPV5...sik/nymphe.mp3
format:
-"magnet:?" starts the url.
-"xt=urn:sha1:XXXXXXXXX": exact topic: sha1-hash, with this the file on gnutella is being described exactly. You will always get exactly that file.
-"&dn=xxxxx": and: display-name=xxxx, with this you tell the program, how the downloaded file should be displayed, till the other name is found.
-"&xs=http:*****xxxxxx": and: exact substitute=http:*****xx. That means, that this http-file will be used, when the hash-download doesn't work.
"kt=blah.blah.blah.mp3": keyword topic. Search for the keywords. Not secure, because you could get any file, which is named such.