This is what planes look like on radar; a dot:
The additional info is either input by ATC when the squawk is assigned, or on all Mode S transponders it is an automatic data feed WHICH THE OWNER PROGRAMS IN.
In other words, the primary return for a 777 looks exactly the same as a 757 or an A340. The "badguys" only have to reprogram the transponder (10 minute job) and be departing from a complicit or ignorant country.
I could go out to my helicopter and reprogram the transponder to read the code for MH370 and the local ATC would not know that I was not an airliner (except that my speed would be too slow).