F.OBN, I'm familiar with these and other, similiar schematics, (eg., Airbus). I see standard, independent peripherals in a typical flight control system with EFIS, and with the FCC in the center. You can't hack this system because there is no "receiver", so to speak, no entry point.
There are no 'bus switches' to 'kill' the autopilot in any airplane I've ever flown, but there are standard autopilot disconnect switches/buttons etc. And if the airplane isn't doing what the PF wants then the crew deals with it as they would any abnormal -take control, ensure aircraft stability, (heading, altitude, and if climbing or descending then level off to troubleshoot), communicate the problem first to one another then to ATC then assess severity, determine and execute a response, secure the airplane, decide next action, advise ATC and request an appropriate ATC clearance.
From what I've seen so far, there is a great deal about transport aircraft in this present thread which is either not being expressed (it ain't a secret) or just isn't understood. It's not that these things are too complicated, it's that there is no way in because the parts are independent of one another, require crew input to function, or don't exist. My point above is to call for some thought on how the B787 may respond because I think it is a different architecture. Until some careful thought is put into this question, no one can make assured pronouncements on that system's security either.
PJ2