> pipes only accept input from MSI signed software

This does not inspire confidence. I'm assuming the pipe exists so that some GUI process running as the current user can perform privileged actions since the other end of the pipe runs as SYSTEM. At this point, just inject a thread into that GUI process and send the command - the service will think it is coming from MSI software, because it is.

The "only invoke MSI signed executables" mitigation is a good one, but if these two things are all they have done, while leaving the "arbitrary registry write" primitive in, then this is still 100% vulnerable to local privilege escalation.