I'm thinking that a web app for resigning the saves to a different HD key might be a solution. It could work like this:
1. User uploads .zip of save along with filling in the relevant HD keys through a web form
2. Save is verified to be already legitimately signed (to prevent cheaters of course)
3. Save is re-signed to work with new HD key
4. Save is sent back to user as a file download
Depending on how complex the signing and verification process is and how much data needs to be transferred, this could be done in one HTTP transaction.
If someone in the know can tell me how to sign (I'm not asking for the key, just the algorithm) I could probably implement it as a web program in PHP using the non-online auth for patched XBEs and hand off the software to someone with the key, who could then change out the key and host it.
In theory that method would be secure, since it would never mean running code that knows the key on a user's computer. The key would remain securely in the hands of those who already know it.
Just thinking out loud here, but if anyone can help me out here, maybe we can get something useful going.