I have tried, and it definitely does work, but it definitely has inferior performance. If you don't shoot demanding subjects it probably doesn't matter. I like action photography and then it's not useful. I can get around with the EF extender 1.4x III mounted between my 7D and the EF 70-200 mm f/2.8L IS II USM, though.
It is simply so, that if you want AF with good precision, then you must have the two sections of the AF sensor far enough apart. You need a long enough baseline. But that also requires that the opening, through which the light is entering, must have a wide enough diameter. Canon's 1D-series have previously had one AF-sensor that sacrificed accuracy to be narrow enough to allow focusing at f/8, and that was one of the sensor pairs in the center. The other pair in the center had higher precision, but required f/4 to work. So from f/4, a camera like the 1D Mark III has a cross-type center point, with different accuracy in the different directions. With apertures smaller than f/4, it had a linear AF point in the center. The other 44 points were all linear below f/2.8, and stopped working below f/5.6.
The 7D, on the other hand, have two zig-zag somewhat higher accuracy sensor pairs in the center, top and bottom, accompanied with normal accuracy cross-type sensors at the other 16 positions. The 7D also has cross-type high precison AF sensing in the center, something which hasn't been available on any 1D camera prior to the 1DX. With this arrangement, the 7D provides 19 selectable cross-type AF points all over the AF area already from f/5.6, and provides high accuracy focusing at the center in both directions from f/2.8.
A 1D Mark III has 19 selectable points with linear focusing only at f/5.6, cross-type at the center (with high accuracy in one direction) from f/4 and cross-type for all 19 selectable points from f/2.8, but even then all cross-type points are hybrid points, with high accuracy in one direction and normal in the other. It took until the 1DX to provide cross-type high accuracy points in both directions in a 1D-series camera. The 1DX has five of these points. But then it should also be noted that when Canon now have improved the accuracy of the AF in the 1DX model, just like the 7D it lost the ability to focus with lenses slower than f/5.6. And even before, one should be aware of that the AF capability of a 1D-series camera at f/8 is equivalent to what the EOS 650 offered in 1987, i.e. one single linear AF point. More computing power behind it today, of course.
