For a long while it was a mystery. (read: the coaches and athletic department did it on a case-by-case basis and didn't feel the need to constrain themselves with externally published rules) The only certainty was that a player graduate before the banner hang.
A year or two ago, when JJ & Shelden's retirements were announced, K went on record saying "national recognition" was a requirement. In practical terms that meant NPOY / NDPOY for our two stars. Could a 1st-team AA who didn't make any of the POY lists qualify in the future? IMO, yes. But we'll have to see.