I earlier commented about there being access to the E/E bay located outside the cockpit and that this was questionable. However, it makes sense after reading this:
"This access is potentially needed for extreme emergencies, such as by the cabin crew to fight an E/E bay fire."
Question is do you put possibility of emergency above chances of sabotage...?
At the risk of adding 'facts' to this discussion.
Many airlines have chosen (or been mandated by their local CAA) to lock the MEC access hatch from underneath. It is often a bolt that requires two spanners to undo it or it can be a spring loaded shoot-bolt. I have seen both used on the same aircraft type by the same airline (not Malaysian).
Opening one of these locked hatches from the cabin would not be a quiet affair and would need considerable effort.
On the other hand if someone did manage to conceal themselves below until the time was right, the hatch is usually covered by a velcro sealed carpet. Not easy to push open from below.
Sorry to butt in.. I'll put my tinfoil hat back on now.