Hi Eric! As much has I appreciate your competence and your attention to detail, is it not possible (or even plausible) that insidious code such as that found in The Underhanded C Contest might have been passed in under your nose?
Of course, it is reasonable to assume that the SE Linux code would be especially vetted for backdoors, and thus other areas of the kernel might make for less-eyes-looking-for-issues cover for a backdoor. But considering how much code goes into the kernel, is it not possible that some innocuous-looking code may have gotten through?
Of course, if SELinux or any other component is compromised (or the hardware), then it is safe to assume that _no_ operating environment is any better off.