(but this did not prevent some well meaning halfbrains from sending additional coins to that address).
_Those_ coins, however, are spendable.
Does not they have been "fused" to 1A1zP1eP5QGefi2DMPTfTL5SLmv7DivfNa address and is spendable only with private key matching this receiving address? And since none can send 51 BTC (coinbase + Your "spendable" coins) out of this address they all are lost?