I highly doubt an OS or an application would be writing to the unencrypted boot sector or an unencrypted boot partition. First off, the OS should disallow that

I highly doubt an OS or an application would be writing to the unencrypted boot sector or an unencrypted boot partition. First off, the OS should disallow that operation without elevated privileges. I don't know if Windows or OSX follow that guidance. I'm not advocating against "full" disk encryption. That's what I use. I just wanted to say that the OP doesn't need boot volume verification and that Bitlocker is sufficient for his needs.