earn your card
Mike,
No idea how they do it now but years ago in pro rodeo you had to earn your card. Back when the earth was still nearly flat you had to earn a total of $1000 dollars in not more time than two consecutive seasons if I remember correctly. Numbers would have to be adjusted but I think having someone document gross open tournament earnings of maybe between $15,000 and $25,000 in a period of two years exclusive of calcuttas would work. They would simply have the venue fill out a form whenever they placed in an open tourney. No need for someone to chase only certain qualifiers but at the same time set the bar high enough that someone can't become a pro after winning the local bar tourney three times.
The number should be set so that a shining star could become a touring pro in a season or less and a hard working and very active journeyman with decent skills could earn enough in two years to get a card. Then you set a bar to keep a card. If that goal isn't met for two or three consecutive seasons from winnings on the pro circuit, then the player has to requalify again through winnings just like the rookie did.
Of course if your pro circuit really got going you could possibly have a junior division to qualify in later that most pool halls could hold events for and the money earned in these events would decide if a player earned a pro card.
Top of my head thoughts but the details don't really have to be complicated.
Hu