Mike - I hesitate to join in here because I am not a coach. However, I can tell you that after seeing several tournaments and events that what is being said about players self selecting is correct.
There are several very good players that never go to a PG showcase. However, very many of them go to at least 1 or 2 of the tournaments. Not going to a showcase can be indicative of a lot of things including choosing to pursue other showcases, lack of financial resources (it is expensive), lack of opportunity for other reasons, football or involvement in other sports, lack of interest, etc.
Still, I'm sure that you're good at spotting talent and potential since you're helping recruit. I've known several kids who know, and their parent know, that they will not play D1 ball. These kids will often still play on competitive summer teams but I've found that many of them choose to focus on smaller schools and target the coaches of the schools they want to attend. They are good players that have chosen not to put much more into getting noticed because they may not have much chance of appealing to schools outside of their region.
The kids I've seen that fit this mode would probably rank in the 5-7 range if they attended a PG showcase, in my estimate. There is usually something that they do or don't do or some physical stature that would eliminate them from playing at most D1 programs. Being 5 foot 6, running an 7.6 sixty as a middle infielder, or having less developed footwork or hitting mechanics, for instance.
I'm suggesting that a lot of the kids you may want to target would be those that PG has on file but that haven't been to a showcase. Some you will find on a state by state list of unranked players, for instance.
I agree with the earlier posts that stated you should go for the best players you can find. I know plenty of 7-8 rated players that have decided to play D3. If you contact kids and advertise you may be surprised at how many would consider travelling well outside of their region to attend your school.
It would be great if PG gave ratings to those that didn't attend a showcase. But in a sense they do that through their national and state rankings. You may get more out of looking there to start.
Good luck.