So...after countless hours spent reading other posts, daydreaming, and searching junkyards and ebay, here's what I came up with:
1. Start with two driver side factory seat brackets from any '72-'75 CJ.
2. Cut off the turned-down part that bolts to the transmission hump on each.
3. Cut the welds that attach the flat iron to the tube (one bracket only).
4. Re-weld the flat iron to the tube, only in the opposite direction.
5. Weld the two tubes together, using a 1" I.D. tube section as a sleeve.
6. Bolt two sections of 2"x2" angle to the tub in front of the wheel wells.
7. Bolt flat irons to the angle and tubes to the door sills.
Interesting note: the original seat sliders and newer '76-'86 sliders (Bestop or Rugged Ridge sell them) bolt directly to the '72-'75 brackets with zero modification (11" center-to-center on the mounting studs). Any new replacement seat will bolt directly to the newer '76-'86 sliders with zero modifications.
My pictured version is slightly more complicated than the description above, since I welded the flat iron sections to some 1" I.D. tube which sleeves and rotates freely over the factory tube, allowing the passenger seat to flip forward as originally intended. If I ever find a set of original low-back seats, and figure out some sort of latching mechanism, I can still have this original feature. For right now, everything is bolted in place.