Yes. The correct way to implement this kind of distribution model is to pick a block number (eg. 290000), and say that bitcoins at that specific block are what gives you aethereum. Otherwise, you can claim some AETH with your BTC, deposit and withdraw from an exchange, claim more AETH, and repeat ad infinitum. For the system to be trust-free and secure, it would have to be based on private key signing, which means that you would have to have them in a personal wallet.

If you don't want to set up a new wallet and deal with different addresses another somewhat technical alternative is to use my pybtctool toolkit to generate a brainwallet and put your money all in the one address just before the nucleus happens. Pybitcointools/pybtctool includes Electrum message signing functionality with the "ecdsa_sign" command; here's an example session:

You can independently verify that 04334... is the public key corresponding to sha256(cow). Then you can easily sign a transaction to move funds back out. It's literally as easy as a command line utility possibly can be. But I personally would recommend for this group to do something similar to what we're doing and release an application which people can send their BTC to (it's client side so no security risk involved) such that the application signs at the appropriate time and then sends the BTC back out. Having easy-to-use tools for doing this kind of signing would really do a lot to help Bitcoin-based issuance models gain wider adoption.