This is what all surveyors in the US use. It has been requested and overlooked for too long. Pix says that this is for surveyors, so… This is a huge overlook.
I think what you’re looking for is available in PIX4Dmatic, might be worth to give it a try. Here is a screenshot that shows you can select from “m”, “ft” or “ftUS” for the Arbitrary Coordinate Systems. You can download the software here: https://www.pix4d.com/download/pix4dmatic. There is a free trial.
Another advantage is that you have all the geoid support for the US in PIX4Dmatic such as GEOID12B, more about that here: https://support.pix4d.com/hc/en-us/articles/360044174411-How-to-use-vertical-coordinate-systems-and-geoids-in-PIX4Dmatic
Let me know if you try it out.
Thank you Pierangelo!