I added support from the TPM to SeaBIOS and have parts of the BIOS functionality successfully running on a Chromebook Acer C720 (as example hardware). Here are some findings on the Acer:
The TPM is successfully detected but sending TPM_Startup(ST_Clear) to the TPM fails since either coreboot or some other firmware seems to already have initialized the TPM, which is fine, and also extended PCR 0 with at least one hash. Ideally there would be a TCPA ACPI table containing information about what was logged, since otherwise the state of the PCR seems not that useful. SeaBIOS's TPM extensions could then also use this TCPA table and add its own logs into it along with extending PCRs in the TPM. So, in this case the TPM SeaBIOS extensions don't log anything and adding additional ACPI tables to the existing coreboot tables seems 'impractical'. I was wondering if coreboot could add such a table if a TPM was found to be present?
An older version of the SeaBIOS TPM patches can be found here (for now):