mmpool.bitparking.com used to do anonymous by address but when I switched to merge mining I needed a way for a user to provide all the alt coin addresses so I switched to needing to register where you supply the alt coin addresses at registration time. It's still anonymous though since it doesn't require any email addresses, passwords, etc.
The other option I considered was having an anonymous address and having the alt coins send to that address too - the user could then import the private key into their alt coin wallets but this was a bit technical for some users.