Nice work @Pat. Clever, well conceived and executed – and definitely worth the significant effort you obviously put into it. For longer, complex documents your approach will help a lot of readers.
I didn’t test this – but since you are using LaTeX for its PDF parser, would your export technique render LaTeX code when our notes contain it?
I would suggest for ease of maintenance and readability, that you consider using a code note and the techniques that Mark Anderson explains here rather than putting the whole text of your Big Rule into the $Rule box in the Action Inspector. That control does not scroll, and reading a rule whose size exceeds the text box is cumbersome.
An alternative to @Pat’s process is to use the Marked 2 application. Pat’s example TBX can be exported to HTML and that .html export document can be dragged into Marked 2 for rendering and saving as PDF. Just another approach for those less technically inclined.
For anyone interested in Tinderbox history, a trip to the legacy forum will reveal some earlier approaches to PDF exporting. YMMV, since that forum covers versions of Tinderbox that might operate differently than v7.