Is there such a thing as an inner taillight panel/brace? Was looking at my car ('73 hatch) recently and noticed mangled metal I had missed before.

My 71 hatch had the full width inner brace panel.  My present  2 door 72 sedan only has a narrow rear brace just wide enough to hold the trunk latch, ~8" wide.  Just speculating, the 2 door sedan cross brace below the rear window where the trunk lid hinges bolt on (speaker shelf), takes the place of the full width cross brace inside the tail light panel on the hatch model.  From a 12-1-1974 repair manual (see below), the 71-73 hatch full width inner brace panel part number is D2FZ 6440492A.  Click on photo for a clearer view.

I would put an ad in the wanted section and hope some one's part's car could sacrifice this part for your project.

From your previous post, you mentioned back halving this car.  If this still is the plan, your new rear subframe could be made to provide the structural integrity better than the missing inner rear panel.  You would have to fabricate something to hold the hatch latch or just use hood pins on the hatch.  If you are going to retain the stock interior plastic panels, you will also need to fabricate something for the rear cross plastic cover to be mounted to.