I think the idea that there are players undiscovered or unused by Cork hurling selectors is incorrect. I think in general the best players are selected and in particular the players best suited to the step up in standard.This is the mistake Cork selectors are making repeatedly over the years. They are going with guys who were succesful in some area of underage but forgetting the time gap before they make inter-county mainly because of the S&C process they are being put through. A guy of 26 or 27 standing out for his club wouldn't have to go through the same process and mostly they get overlooked.
We saw four players brought into the squad two years ago and none really made the step up. Cork selectors and management probably go to more games than anybody and are more acutely aware of what is needed than anyone as they work with elite hurlers all the time.
Personally, I think any player that has not made inter county by 26 is probably not going to make.... there will be exceptions but I would guess they are few throughout hurling in every county.
I don't know for certain but I would expect that a lot of the better club hurlers will have been included for Cork at times to see how they perform without actually becoming part of the squad and most will probably have played against Cork teams in trials and challenge games.