Whatever the unknown requirements are from Valve, Crimson Gray was just recently allowed to submit trading cards! After a little work, we have the complete set accepted and now released. So if you want Lizzie to grace your profile, want to see some previously unreleased John concept art, or just like collecting cards... now is your chance!
Note that you should receive drops for time already played. For most of you, that should mean that you receive the whole set of cards immediately.
Though the requirements remain a mystery, this is a positive sign for my other games. It may not happen immediately, but they are likely to be given cards in the future. Rest assured that if a game is permitted to have cards, we will make it happen!
Expect the next update here to be a preview of the DLC expansion to Crimson Gray. ^-^