The traditional model for humanist typography has been the letterforms of a right-handed scribe with a broad nib pen. Are there any humanist typefaces inspired by the letter forms of a left-handed scribe with a broad nib pen?
This answer may sound like I am not answering your question, but I am. It's just probably not the answer you expect.
It's the angle the nib is held at that makes the strokes thick or thin in hand writing/calligraphy with a broad nib - typically around 30° to 45°.
The angle of the nib doesn't depend on whether a person is right handed or left handed. I do calligraphy myself, and I'm right handed, but it wouldn't matter if I was left handed. The letters would look exactly the same. Left handers do have to hold the pen differently however to achieve the same angle. There are several techniques such as writing overhand, or rotating the paper, also strokes can be done in reverse, from bottom to top, so you aren't pushing the pen forward against itself.
There are also aids to help left handed calligraphers achieve the proper angle of the nib, such as left-oblique angled nibs, or angled pen holders, but these aren't absolutely necessary.
So, basically there would be/should be no difference in how fonts are designed, whether by left handers or right handers.
Without prejudice to the above answer, Bringhurst does emphasise the right handedness of the terminals and letterforms which inspired humanist typography (see the illustrations in Elements of Typographic Style). A bit of internet research discloses only one typeface consciously based on a left-handed scribe, 'Left-handed Gill' a modified version of Gill Sans based on the strokes of a left-handed penman; other typefaces may well exist, but this is the only apparent one.