I think the coaches see these guys a lot more than you give them credit for ’90. In one of the articles about CJ this year they described Reed coming to his house despite an intense storm for his in home visit.
Also, AAU events give coaches a great opportunity to see 1,000’s of kids live in a few days. Oftentimes more than once. For example, the EYBL which is the best of the best of summer ball has 4 sessions a year in Minneapolis, Hampton, VA, Oakland, and Dallas – http://www.nikeeyb.com/category/eybl/. Hypothetically, Stephen Ott (who I believe was the lead recruiter on AP) could have gone to Minneapolis and seen him play live 3 or 4 times in a weekend. Add in another midwestern swing in the fall to drop in on open gyms at a few different high schools, and I think it is conceivable that someone on the staff saw AP play upwards of 5 ot 6 times live. And if you told me it was over 10, I wouldn’t be shocked.
Kempton is a little tougher being from farther away, but his team I believe played in the Super 64 in Vegas, which I would be shocked if 90%+ of Division 1 programs weren’t represented at. The staff could easily have seen him there, and maybe gone to an open gym at some point once they knew they had a shot.
There are also the academic showcase camps like this one – http://www.eliteacademicathletes.info/Exposure.html – that target high academic schools with smaller recruiting budgets and pools of players. It allows PL, Ivy, and other schools to look at kids who they can get into school, instead of wasting their time watching random aau events in which a large portion of the kids don’t have the interest or grades to get into PL schools. (check out how they spelled Lehigh in that link for a laugh, but do notice the other like minded institutions on the list).
There are a bunch of ways for coaches to see players, and if a coach wants to keep his job, I would hope that he and his staff watch every kid they are bringing in live (hopefully many times) before they pull the trigger. Not having seen a player live would be akin to a manager hiring a team of people without ever interviewing them face to face, and no one in their right mind would bet their livelihood on that.