I'm exploring a sensor system that gathers information about gait and changes over time, as well as identifying precursors to falls, the primary focus was an insole, but a secondary was in socks and was pointed in your direction.
Lots of running shoes and boxing boots are very light these days. I wonder if there is an opportunity to work with one of these companies to incorporate some of their approaches into a sock or light weight shoe to advance your suggestion.
Something like http://uncrate.com/article/zemgear-360-ninja-split-toe-running-shoes/ might be able to be built on to include additional stabilization through the ankle/lower leg. Even paring this with an active ankle style brace to prototype http://www.activeankle.com/
I like where you are going with this! I think this paired with video assessments (likely done with professionals of sorts), the sensor insoles I am bashing away at, and I am sure other things have great potential to minimize risk early on. You could have an interesting core idea, almost a platform, that could consume data from different systems to take a comprehensive view of the current risks and suggest mitigation approaches that are more 'accurate' as additional inputs are added. Perhaps the base being a questionnaire, and build in features of the phone, the next level could be a video of the person walking with analysis of gait etc that you mentioned earlier, then sensor insoles, and perhaps professional video assessments etc.