The thread title asks for evidence that isn't there - or it would have been shown many years ago. What
can be shown however is some circumstantial and coincidental 'evidence', and that is the reason that the question is being discussed in the first place.
First of all, I find it disappointing that both pilots and laymen in here find it hard to believe in even the technical ability to remotely fly a large plane in 2001. With that mindset it is obviously hard to move forward to consider the use on 9/11 if it is thought that it's not possible in the first place.
Even cursory research reveals that remote control of a large Boeing had already been developed many years before 2001. Here is a wikipedia entry showing that a test in 1986 involved a remote takeoff and approach to impact. Such flights were most likely still pilot operated, just remotely, and not pre-programmed computer routes so that an FDR would still show minor jiggles from hand flying. As did the Pentagon FDR.
http://en.wikipedia.org/wiki/Controlled_Impact_Demonstration
Now that the hurdle of the technical possibility has been overcome we can move on to consider the next stumbling block. That of actually taking over control from a sitting pilot/highjacker. As the physical link between a pilots hands and feet have been replaced by 'fly-by-wire' servo systems that possibility becomes much easier. All of the pilots input is now intercepted by a bank of computers and relevent information passed on to the control surfaces electronically. The location of the pilot is now not important. Obviously in normal flight he would quite like to be able to see out of the front window but 'technically' he could be sat at the rear and observing a TV screen for forward vision. Indeed, if there was a sufficienty fast and reliable radio link he could be sat in a ground office doing exactly the same job. Remote flying. Such a link was already available as demonstrated by the 1986 event.
The next item requiring attention is some kind of radio enabled switcheable modem that would allow all control to be moved from one set of controls to another set so that total control can be switched between any two pilots. Once control is taken from one pilot none of his controls would operate until the modem switched back to him. What if the second control centre was on the radio link as discussed before?
Such a device has been long discussed as an anti-highjack measure and suggested as a means of removing power from a highjacker and landing the plane where the authorities desired. Again, there is much evidence that Boeing has already installed such an interface without asking their clients and law suits have ensued ending in large fines.
We now have the ability to remotely fly a plane and also theoretically the means to take over total control from the plane itself and cause everyone on board, including crew and highjackers, to become passengers. From then it would be possible to interupt the oxygen supply of course, with obvious results. Crew, passengers and 'patsy highjackers' alike being helpless.
We now move to circumstancial and coincidental 'evidence'.
Circumstantial :-
It isn't widely known but in the year before 9/11 seven senior technicians from Raytheon were taken from their normal duties and seconded to a secretive task based in CA. They were instructed to not discuss any details even with family. ( Grieving relatives revealed this) These people were senior experts in cutting edge technology in the emerging field of unmanned drones and remote flying. (Raytheon went on to become world leaders in this technology and were granted contracts worth many millions if not billions post 9/11.)
Coincidences :-
All seven of those Raytheon experts were passengers on three of the 9/11 highjacked planes and died that day.
Raytheon had an office in WTC2 - 91st floor. Impact zone.
Raytheon also rented approx 9 sq ft of space in WTC1 - some kind of cabinet. Close to impact zone.
Discuss :- What if these Raytheon experts were used to develop and install equipment in the Boeings used in the 9/11 event? What if they had been told that their work was to develop anti-highjack systems which was commercially sensitive, and highly confidential. That it must not be discussed, to avoid a competitor spying operation beating them to this new lucrative market. What if they were honest men who would have been on the 'phone to the FBI in minutes if they had watched 'their' planes fly into the towers. What if they had to be prevented from doing that in an elegent manner. What if, on 9/11, they had been told to fly on those planes as observers of a development test of the system.What if the two Raytheon locations in the towers housed some kind of homing beacon to be used in conjunction with the onboard equipment.
All of the last part is pure conjecture and speculation of course. As 'evidence' it is non existant. But the information before that is relevent to the discussion, and that causes some people to muse over the various questions raised in the 'discuss' paragraph.