Having made a very rugged router sled out of 20mm extrusions, I have a hard time believing that the MFS sticks could possibly be rigid enough for this purpose. Even doubled up, the 20mms will flex a bit in the middle as my 2200 rides over it. Considering the size of the gargantuan bits one generally uses when flattening, the thinner profiles do you no favors--I almost always find myself jacking the thing up just to get it to clear with the bit bottomed out in the collet.
The MFS is a neat idea, but I can pretty much always accomplish the same thing with some half inch stock and a bottle of 2P10 in far less time than it takes to mess with all those stupid little screws.