I guess I conflated the blockchain feature with other things I thought I knew about the protocol. I assumed that the headline Satoshi embedded in the genesis block was done via memo field.
January 3, 2009, Times of London; “Chancellor on brink of second bailout for banks”.
That headline was embedded in the scriptSig section of the transaction. This is where a signature would normally be included to prove authorization to spend the previously unspent output. Since the only transaction in the genesis block is the coinbase transaction, the scriptSig section is unused and unnecessary. Therefore, the space was available for embedding the headline.