futurecatcher ...
Tho I don't even know where one would go to find those numbers (perhaps PGStaff has some info), my gut reaction is that more of the current (American born) minor leaguers went to premiere showcases and played on premiere teams [our son has met and competed against quite a few of his former ... current? ... nemeses (the plural of nemesis, I believe) since starting his minor league career] than the established major league players. Perhaps some of the younger players on the 25 man rosters competed and showcased, but I suspect many of them ... as well as most of the established major leaguers ... played in their local leagues, at the local high schools, and perhaps played at a JUCO or a 4 year school.
Just a gut reaction based on what I have witnessed in terms of the players we have watched over the years, some in their mid-30's now who played at our local high schools.