The problem with these things is that they form again as the temperature of the gases cool so you want the top of the riser hot as.
Regarding flat bottom woks vs round ones, I have to ask if you have ever built a flat roofed pizza oven for your customers?
And of course you get to see the flame so it may be easier to regulate the heat.
As for the apparent lack of health concerns from a life time of cooking over various non optimised appliances, I believe that's called observer bias. In other words, it's only because you haven't had an issue that you're here to post. Those who were subject to respiratory disease etc have given up this pastime and aren't here to post their observations.
Anyway, I think if we are to promote this as a form of cooking, it behoves us to consider the unintended consequences even if we can't investigate them. It may be that an open fire is not worth it and one needs to modify it into a traditional wood stove cook top with the circular pot cut out, and gases vented away from the cook.