This might be in a companion library but Clevis and Tang implement a protocol for automated decryption (remote unlock).
It might be useful for tempel to support at least the client part if not more.
Clevis and Tang provide a way for a system to decrypt secrets if it's in a specific network or has access to TPM.
Seemed quite ingenious to me and worth mentioning in the context of tempel.
Might be useful for starting an application that needs to decrypt a bunch of service (the admin) credentials without requiring user input.
https://github.com/latchset/clevis
https://github.com/latchset/tang