I'm pretty sure they aren't ciphers. My vote is on hashids ( http://hashids.org/ ) like goo.gl and so forth use. I just don't know where they can link to since they don't work with goo.gl or the Twitter's t.co and a couple others. Just matches too well to the format. This would mean that, with a salt, they'd align to an integer value or three (depending on padding). So a database table id or comment id in a forum or a coordinate if 3 or....?
I don't really have rock-solid reasoning but:
1. They're specifically 6 characters like the hashids.
2. They way they're being sent out makes it easy for them to get out-of-order. And without a solid clue that points to the order, that makes it difficult to combine them into a longer string for a sentence or combined url. This isn't quite so solid if we go off of official release dates of magazines and Tweets and such, though.
3. The better idea would be to make each 6 character code a link to an image or something that can then be combined together in some other way. I think this also makes sense from a marketing stand-point since you're solving a little at a time rather than having to wait until the very end. Keeps people going.
4. The 6 characters doesn't fit into 32/40/64/160 or other hash lengths if combined.
5. Like others, I've gone through the different ciphers to see what could possibly match and there and there just doesn't seem a solid match. That makes it less likely to be a cipher since I kinda doubt they'd invent a new one since...
6. ... the codes don't seem to be a relatively easy code. This is a video game, not Kryptos. I'm very much a beginner with this stuff but I dug around quite a lot and tried a few things and can't get a handle on what it'd be. There's no single number in each block (some have none and one has 2 numbers). And the numbers are not always at the beginning (and especially not at the beginning of the first code) so I think that rules out using numbers to shift the alphabet (plus the '25' double number in the latest code throws that off anyway) or define word length. It uses upper and lower case and numbers so we're working with an expanded alphabet. Which, again, that hashids.org uses A-Za-z0-9 as the default alphabet - more ammo in my argument that they're hashids.
So, those are just my thoughts and I wanted to share.