Just won one of the exclusive banners, and it's not showing up in-game for me or anyone else. It shows up on the website fine, but I doubt many people care about that. It's all about showing it in the game.
When i updated, I realised that one of the steps was downloading new banners, what i think is whenever a few are up (maybe tonight, maybe tomorrow, maybe when some sheets are full, the devs will modify the file with all of the requested (and approved) banners, then your updater will download the file and you will see them ingame.
This is because the banners you see ingame are the ones in your file std_banner_a and so on (located in warband/modules/crpg/textures iirc), so you have to download that file again with the updated banners at some point in the future.