Yes, you can only show that you owned something at a certain block height. That said, if you constructed the proof at the start of a gallery exhibit, I would argue that this would be sufficient proof for most people interested in verifying the claim.
For a credit rating, a proof that you had certain credit score at a certain date is likely good enough as well. Alternatively, you could imagine situations where proofs could be constructed in real time but verified offchain (e.g. by showing a generated QR code to another device that isn’t connected to the internet).
I like the time lock idea. I was thinking about whether we could use a similar technique for off-chain transitions.