I have worked with current and former players directly as far as licensing their images and paying royalties. It’s not difficult to acquire a player’s rights per se, the contracts we used were simple and didn’t even fill 2 pages. But it can be difficult to get a response: the effort in following up and communicating with player’s representation can be simple or an absolute pain. In the case of MLB The Show where they have secured a huge player pool including the rights of current MLB players as well as those who participate in the “blanket HOF” licensing, they wouldn’t spend too much of their time on overtures to secure any one particular player, say McGwire or Alfonso Soriano etc.
Not all HOF players participate in the licensing, and have to be contracted directly. This is the case for Mantle, Clemente, Aaron, and many others. It’s an interesting business to say the least.