I asked FOG and Festool directly about that clamping situation about 2 years ago - no solution, unless you make one yourself. One of the Kreg adjustable lever action clamps supplied for use with their clamping station might work for your purpose, since the base of the Kreg clamp is adapted to receive a threaded bolt and the head of the bolt is retained in a U-shaped track profiled recess similar to that of the top slots of Festool's side rails. The Kreg clamps are an adaptation of the original Vise Grip design.
I have encounterd many clamping situations in which I could have used and would have preferred such a clamp because clamping a part of the workpiece that extends over the side rail places a moment on the workpiece lifting it from the table, unless you place a thin (1/2 inch) piece of stock between the top of the side rail and the bottom of your workpiece. I keep a piece of 1/2 plywood handy for that use, in one of a pair of shallow mall accessories boxes that I made from cutoff scraps of plywood etc.
If you dared, theoretically, you could take a Festool L-shaped clamp, place one end in a vice, and heat a portion of the shank with a torch then twist the shank 90 degrees to make such a clamp, then retemper the twisted area. But I don't recommend this, because even if the twist modification was successful, the forces generated upon use of such twisted clamp would tend to twist the upper inside surface of the top channel in the side rail and probably bend it permanently upward. What is needed is a brace that would extend over the outside (vertical) surface of the side rail to react against that twisting motion, and maybe even wrapping around to bear on the underside of the side rail, or a bracket/base member that simultaneously engaged both the top and side channels of the side rail to resist these moment forces. That sounds like a significantly different clamp design to me. Festool: I'll takke at least two of these.
Dave R.