Sliding seats get used all the time. The rules do not say that the brace must be hard mounted to the seat, only that if your particular seat calls for a permanently mounted brace, follow the instructions. Otherwise, you must have something within 6 inches of the seat back to stop it failing backwards.
We have a seat on sliders, and we have a seat back brace with a large plate on it mounted behind. We have a few holes in the brace tube so you can pull the pin and move it if required. We've since found a happy medium where our tallest driver basically drives with the seat just touching the brace, and our shortest sits forward but the brace is within 6 inches, so we don't have to play with it during pit stops.
We are using this style brace.
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