EDIT: It sort of does what he wants I think in that you SPECIFICALLY TELL THE CLIENT THE SENDING ADDRESS. You need to have coins in that address beforehand. So in the OP's original state there would have to be a payment into the address (that is to be used as the recipent) and then send the coins to the other site.
The modified bitcoin client from the above thread allow you to send from a specific address or sets of addresses. You need to enable the anonymity tab and select the address. It can also be done at the commandline with
bitcoin sendtoaddress <bitcoinaddress>[:<sendfromaddress1>[,<sendfromaddress2>[,...]]] <amount> [comment] [comment-to]
I think sendfromaddress1 will be the primary response address, but don't know for sure as I haven't tested it.
I've used this client within the last couple of days to send and receive, so I don't think that it has any problems with the feb20 changes. I believe there will not be an 0.5.2 build or an 0.6 rc1 build - the next will be shortly after mainstream 0.6 is released officially. http://coderrr.wordpress.com/2011/06/30/patching-the-bitcoin-client-to-make-it-more-anonymous/
I did a quick tutorial with with images in the above thread:https://bitcointalk.org/index.php?topic=24784.msg742919#msg742919
For windows: http://mtgoxlive.com/downloads/bitcoin-v0.5.1+coderrr-win32.zip
For linux: https://github.com/coderrr/bitcoin/tree/v0.5.1%2Bcoderrrhttp://mtgoxlive.com/downloads/bitcoin-v0.5.1+coderrr-linux64.tar.bz2
I believe there are other clients that can do this but don't know which offhand.