I have read this thread with interest. I live about 2km from the WTC. The smoke in the air is a constant reminder of the carnage.
I am a CPL holder but fly only privately. I have a M/E instrument rating and am familiar with the 767, having read the manual and used certain FS add-ons. I have also logged 1 hour in a B744 sim. I was able to get a the sim off the ground, fly a 10 mile circuit and shoot a reasonable ILS, with nil wind or turbulence, and some coaching on EPR and flap settings. Once I had become used to the operation of the trim on the sim, the a/c was fairly easy to operate. I believe that if far enough out, it would have been fairly easy to guide those 767s into the WTC with the A/P disengaged, provided the person had a reasonable amount of GA and probably twin experience. It is now clear that the hijackers had this.
Now, landing the a/c, managing fuel, airspace and assymetrics etc would be a very different matter. Indeed, operating the FMS would probably have been tricky. But they didn't need this. People have talked about having a knowledge of the navaids in the area - but I think the solution would have been much simpler. An $800 GPS on the glareshield with a preset waypoint for a point a few miles out in a straight line from the towers would have been all that was needed together with a basic understanding of the MCP. Get the heading from the "goto" button on the GPS (wherever they were when they took over the a/c), spin up the heading and a gradual descent to 2000' on the MCP and I am afraid that would have done it. With the aircraft more or less in trim and given their GA experience, hand flying to the towers from this point would not have been too challenging.