For period events, I recommend hobnails. If done correctly, they are perfectly comfortable, safe, and period correct. I have a problem with modern shoes only if you are attempting to have a historically accurate event. If this is merely themed shooting, I am okay with costumes of your favorite westerns regardless of how fanciful. For those events, I'd have no problem with showing up in my modern "Cowboy" rancher gear (Modern Ranch shirt, Wrangler Jeans, Ariat Boots and a modern Stetson and an Arvo Ojala holster.)
However, I do completely agree with Niederlander that safety should be the primary concern in a live fire exercise. For that reason, the event coordinators should be held to their obligation in providing a shooting course that is as free from slip/fall hazards as common sense permits. For everything else such as mountain man live shoots, or primitive hunting, I've found period moccasins or hobnails to be an adequate countermeasure.
-Dave