Just a couple of days ago I mentioned a similar concept in another thread, which is much simpler but would cater to a slightly different use case: a cold wallet which you should be able to "forget about", even if quantum computing becomes a thing, but it should be possible to be accessed at any time.

With your idea it shares the concept to use a hashlock, but in this case as an additional requirement to the ECDSA key. This means to spend the coins you must know both the private key and the secret.

However, there's a flaw in this concept, and this flaw would (if I understand your setup corrently) also affect your solution: It doesn't at all protect against the "short-exposure" quantum computing attack, because the secret (the preimage) will be revealed at spending time. This means the QC attacker can still replace and double-spend the transaction if it's in the mempool. And against the "long-exposure" quantum computing attack, simply never spending from cold wallet addresses is enough.

This means, the hashlock does not lead to additional security, but instead to a potential vulnerability. I get that you mean that as an "emergency backup" and is meant for lost keys and not necessarily as a post-quantum mechanism, but you have to take this potential risk into account.