It is not possible to actually 'seal a private key'. First of all, it is basically impossible to safely create pre-funded collectibles (safe as in: the creator has no access to the private key).
The method Leo mentioned, allows you to make user-funded collectibles safely, but that kills the coin's collectible value, as now the user who funded it, can scam a future owner of the physical coin.
In my opinion, for substantial amounts and as technology advances / gets cheaper, hardware-wallet-inspired collectibles should be considered.
For instance, it is today possible to build a device which uses an open-source 'avalanche' circuit to generate entropy and private keys and store them in a secure chip, which even the creator cannot extract, although he has hardware access to the device. The device would be able to display a Bitcoin address, though, and destruction would be necessary to spend the funds.
This is all possible and mostly how a hardware wallet works (only really changing the way the secure element grants you signing access), but those aren't exactly cheap, so this is the one drawback.
What if there is tool which can add up the password to both public key and private key information. For example, whenever BIP38 gives 58-character Base58Check-encoded printable string it should also scramble it with 2nd layer of characters which could serve as username or characters associated with some password.
So that will be printed on the physical note or coins, the first owner will have the password and X-character layer which are neither public keys nor private keys thus, no auctioneer or collector will ever know what's inside unless and until they access the info inside.
I dont know, this could be very simple addition for such big problem since many years when it comes to passing down the physical coins/notes.
