simple answer... police will refuse and the courts will back them
if this was done and proved to be inaccurate it would set a legal precedent that would see thousands of fines being overturned.
when they follow you and track you for x time and x distance using their speedo as a reference and blah blah, this will stand of course...
but in cases like one forum member here who drove past an off duty cop that was in a stationary vehicle, who just "decided" he was travelling at a particular speed that happend to result in a loss of lisence... at 3am... that is simply not possible
police aren't superhumans, they are just ordinary people in a blue uniform. There is NO WAY they can actually work out speed without the use of cue's and some form of indicator. it's not physically possible.
I'd say with enough money spent you could get it done... but they will fight tooth and nail to stop you