[dist-epoch]

> If you have to put a password in before boot that needs to be combined with the TPM key to unlock the drive, it would help in scenarios where a TPM key can be found later.

Bitlocker already does this if you use a PIN/password.

[highwaylights]

You might know better than I do, but I had believed that Bitlocker used TPM PIN when you use a PIN, which is challenge/response (i.e. if PIN matches then TPM releases key) so wouldn't help in this case.

If Bitlocker PIN is split key then yes that would be ideal, but I think you can change the PIN without re-encryption (which implies it's challenge/response).