Seat Support

You could make an adjustable brace.
Square tube with the brace on one end.
The brace tube slides into a larger square tube that is welded to the cage.
Use a large pin to secure it.

It is essentially a receiver hitch with a brace instead of a ball.

KT

It depends on how high the seat back goes.  If it it at the middle of the helmet or higher and you have done a good job disabling any recline feature, then you would can use it.  I would advise a racing seat though, becaseu the oem seat is going to make you a lot more tired and isn't as safe.

Wow I must have been really tired when i responded to this, because my response in no way answers the original question.

We have a fixed back Sparco FIA seat, but the center of the seatback has zero structure.  It is literally fabric covered foam and nothing else.  A seatback brace would indeed impale the driver unless it had a very large plate welded to it to spread the load to the sides of the seat.  Luckily, our seat is within 6" of the crossbar, so we could leave the sliding portion of the brace off.

Our back/seat brace is  a short section of 1.5" DOM welded directly to the rear hoop structure. The brace piece is a large section of plate steel set at the same angle as the seat layback and fitted to accommodate our tallest driver.

I'll take a picture if you need it.

KT

We have the same seat, but we modified the I/O port adjustable brace.  Basically, we welded a large cross piece to the actual brace and then welded that to a long, flexible vertical piece connected to the seat brace.  We wanted to distribute force along the greatest possible area.  For the next race we were considering expanding the cross piece with a large place to increase the overall area even more.

The I/O port system with similarly reinforced plating could help with an OEM seat.  However, the comments about increased work = increased fatigue = more danger is completely valid.  At Nelson Ledges we raced in some sorta Detroit style bucket seat and my left leg was exhausted after my stint.  When it wasn't working the clutch, I had to brace *hard* against the sidewall.  The next race we had the relatively well-sized Sparco - I was much less tired.

Thanks trekkor!

I'll let Nick answer officially, but we have the same set up, and with the shortest driver in the car, the seat is within 6" of the horizontal bar on the roll cage. He told me that was okay and it has passed tech like this twice.

We are using an Ultra Shield Spec Miata seat (but no it's not in a Miata so stop hatin'! )which is mounted to a set of stock seat tracks, we have to have adjustability because we have a large driver size disparity as well. Last year it passed without a rear brace, dunno about this year. What say you, Nick?