Mounting a chunk of rail on top is possible. You would have to make cuts in the slide cover and at least drill some holes in the stamped slide. Replacement parts are not easy to get nor would they be cheap if they were easy. Most folks don't want to cut or permanently modify their gun for this reason. Also the rail could hold heavy optics which I don't think would do any good for the system and be hard to keep mounted. There is not much meat or width on the slide to mount a full rail with heavy optics it would be difficult to keep the rail mount from breaking or moving.
If parts were reasonable I would weld an adapter for the mini red dot down low at the rear of slide instead of higher up on an adapter made to clear the slide cover.
there is a rail that slips on the bottom of your lower rail turns up the side then over the top. this attachment is strong and thin thus allowing you to still use your lower rail. I know this because I have mine set up this way. with a C-more on the top rail and a light which I wil be changing soon to a light/laser on the bottom rail. Bob at FN stated this would not void my warrenty since nothing is milled off. Pat (H2oStone - waterstone) it is made for a go to Smith&Wesson gun but its not on there site I will put site up when I get it. http://www.ncstar.com/index.htm