I can't think of any legal way you can do that. - GA
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GCR 9.4.2.D.1 ...Any number of additional reinforcing bars are permitted within the structure of the cage...

Put horizontal bars as low as possible 1) from the base of the main hoop on the driver's side to the base of the front down tube 2) between the bases of the main hoop (across the car) 3) from the base of the main hoop on the right side to the base of the front right down tube and 4) across the car from 1) to 3). Place 4) so that the front of the seat mount can be secured to it. 5) Depending on the configuration of the floor, attach another tube from 2) to 4). The two longatudinal tubes - 1) and 5) plus the lateral 4) - give you three sides to attach the seat bracket to. If the seat attachment tubes are too high, fab a bracket that welds to the top of the tube and drops down to the bottom of the seat bracket.

Since the additional tubes do not touch the floor or frame, there are no additional attachment points. And the seat attaches only to the cage.

Now, I am making no claims about the additional weight. If your car is at minimum weight, this should not be an issue. If I were to add weight by adding cage tubes, I could lose about 40 lbs with lighter wheels.

Hope this helps.