Is anybody using the GDT workbench? https://github.com/juanvanyo/FreeCAD-GDT By going through the release notes of 0.18, I just became aware this exists.
It seems to implement ISO 16792 for geometric dimensioning and tolerances. GDT seems to insert the symbols directly in the 3D viewport. I guess it would make sense to implement those symbols directly in TechDraw.