can someone select a private key and perform an action (such as creating a specific transaction or something else) that effectively claims or reserves that key?
Once you create a private key, it is completely yours forever unless you expose it to someone else intentionally or unintentionally. There is no possible way that someone can generate the same private key as yours in a life time and FYI you no need to create any transaction to do that, all you need to do is generate a key using the mnemonic seed and generate a corresponding bitcoin address for it.
But don't try to create a private key on your own, the randomness of human genrated generated priv keys is expected to be weak.