You can mitigate this by including PCRs that sign the kernel and initrd, however it means whenever you update you need to unlock manually. On Redhat-based distros this can be done with PCRs 8 and 9, though IIRC this may change on other distros.
Also AFAIK there is no standard way to guess the new PCRs on reboot so you can't pre-update them before rebooting. So you either need to unlock manually or use a network decryption like dracut-sshd.
You can use tpm2_policyauthorize and allow the PCR to change without having to manually unlock. This was not supported in TPM 1.2.
You can use it with Systemd.
https://github.com/tpm2-software/tpm2-tools/blob/master/man/...
I think it's doable by dynamically creating lambdas based on test cases I define in one way or another, perhaps like mocked integration services, that does nothing but validate if the event from SFN matches a schema, and that the mocked response also matches a schema.
My concern is that I can't find prior projects doing this. My use case is mostly (exclusively at the moment) calling out to lambdas, so perhaps I can get away with this kind of type checking. But it's just weird that something like this doesn't already exist! Past experiences have taught me that if no one have tried it before, my idea is usually not that good.
Let me know what you think!
(Would have liked to use durable execution which totally solves the typing issue, but can't in this case)