Being active Army wouldn't matter. The Marines as DOD proponent provided the instructor training to other services (mostly Army) and those instructors in turn could train any number of units. There's also the fact that the POI used to be easily obtained from the internet so even units lacking a certified instructor could, with competent NCOs, conduct most of it to standard. For a mission like you describe, a certain number of qualified instructors would likely have been mandated to conduct the training. That's actually the easiest way for an Army guy to get the Marine school - when a JCS order mandates it. The Marines used the school to train instructors for their MEU's. Typical pre-OIF MEU would have one company trained for riot control. Over the years a number of Army MPs have gotten the instructor course so the knowledge is pretty well populated out there now. I believe Marine MPs have the lead for this within USMC and the instructor course is at Fort Leanord Wood where Army and Marine MP schools are collocated.