Seat back brace

No, vertical. Like a stripper pole centered behind the driver seat, welded to an extra floor bar at bottom and an extra roof bar at top

I didn't realize it was this simple!  Could you then theoretically attach the harness to the extension?
The reason I ask, my plan was to use a ford ranger with the main hoop positioned where a pickup typically has a roll bar, towards the front in the bed.  then have the front hoop inside the cab like normal Lemons with all the door and roof bars coming thru strategic holes in the back of the cab.  So a seat brace could also come thru a hole, but I wouldn't want to run harness straps that way.

I agree; I wouldn't put the harness on it. I think I've seen harnesses mounted to a wide,shallow, forward-facing hoop, which is essentially what jimbbski is describing except with bends at each end instead of separate tubes, but I'd check with Pagel before proceeding. Either a hoop or the use of two support tubes would be a more elegant solution than our "T" anyway, even just as a back brace.

Edit:  One other consideration is that if you anticipate enough range in size of drivers that you need an adjustable brace, I'd say just buy one as suggested by Brett85p, above. I don't imagine it's worth the time and money to make your own that would be good enough to satisfy tech, then still have to argue with tech that your homebrew solution is, in fact, good enough. In your particular case that still wouldn't address your harness question, though.

KIng,
If you are running a Ranger will your seat even be 6" from the back wall of the cab?

3.F.1.c Seats Without Seatback Braces. If a seatback brace is not used, a strong,
seat width element such as a shoulder-harness bar must be located within six inches
of the seatback to prevent the seat from failing rearward.

Full disclosure: Novice, no experience.

Are you dead-set on building the Ranger? Is that the only vehicle you'd consider building? Because there are plenty of inexpensive vehicles available, and likely a lot more coming in the near future.

I don't think an NBA big man could safely race a Prius based on internal clearance issues. But that's just a guess. If the Ranger is too tight for you, perhaps another choice?

Driver's helmet clearance from the cage, and padding requirements, is addressed in the text of the Rules.

Several users here [with a lot of experience building Lemons cars] have suggested to place the seat _before_ building the cage. I have adopted that when planning on a design for a cage. The reasons given in several previous posts and threads are compellingly logical.

There are many threads here about building cages and cars which have a ton of information.

With out knowing if this is Lemons legal.  What about installing the main hoop outside of the cab just behind the back wall? You would have to run the tubes that connect to the main hoop through the cab sheet metal,  such as the halo bar but unless I'm totally wrong why would this not work?

The other issue with a vehicle like a Ranger is that it is a "body on frame". You will have to construct outriggers off the frame so that the tubes connect to something other then the cab floor.