Based on my rudimentary understanding of Public Key Cryptography, here's the scenario that I envision:
Current Genesis block is aware of your public address and how much stake you have (both for AGS and PTS).
By submitting your private key, it proves that you donated from that public address, so your stake is known to the superDAC.
The superDAC could then output the stake amount and an ID code for the new DAC, and sign it with your private key (proves that it is you) as well as the superDAC's private key (proves that the stake is valid).
The signed message could be verified by the new DAC for people claiming genesis shares. The only requirement would be that they have your public address as well as the public key for the superDAC.
Okay, after looking at Xeroc's links, those make some sense and wouldn't require a 3rd party.
How is it that the pubkey isn't known in the genesis block? I thought addresses were pubkeys?
If that's the case, it's easy for the new DAC to generate a passphrase, encrypt it with the pubkey so that you can only decode it with the privkey.