The claim that SMM is measured by (Intel) DRTM is interesting. Do you have any details on that? To my knowledge Intel was trying to solve this issue using the concept of an 'SMM Transfer Monitor (STM)' not simply by measuring the SMM environment [1]. But it's been 8 years since [1] was written so if you have links to more current information, it'd be welcome.
[1] https://blog.invisiblethings.org/papers/2015/x86_harmful.pdf...