Of course you can communicate with the OS. The disk is unlocked at this point, not that you’d want to use persistent storage for this. You’d put the info in RAM

Of course you can communicate with the OS. The disk is unlocked at this point, not that you’d want to use persistent storage for this. You’d put the info in RAM. Doing this in a secure fashion doesn’t seem very hard. For example, hash the password in the same way the OS does. Hash that hash with some salt. Put it in memory at some agreed location. The OS can retrieve it, verify the hash and proceed if it matches. And of course erase the message. If you’re worried about replay attacks from someone who can capture the message between login and the OS verifying it, you can add the current time to the hash. If you don’t trust the clock, I’m sure the TPM can produce an ephemeral nonce for both sides to use, or the OS could store a hash of each message after verifying it, and reject any reused message. You’ll never be safe from software vulnerabilities, but this reliably prevents an attacker from reading your disk from a cold boot without knowing or cracking your password.