Roit PLS!

You can get keys from ARAM and Twisted Treeline! You'll need to earn Chests more strategically though. Check out the Hextech Crafting Guide for more information!

Keys, perhaps unsurprisingly, open chests. Key fragments will sometimes drop after victory in any matchmade game mode--unlike chests, you can earn keys in ARAM or Twisted Treeline. Three key fragments can be forged into a single key; you can also purchase keys through the store for RP. Key fragments drop a little more frequently when you play with friends!

Has anyone actually gotten a key/frag from anything other than SR?

I was under the impression that they plan to release it, but it's all tied into the champ mastery system being implemented on ARAM and TT.

You'll have to wait for Riot to finish up implementing Champ Mastery for the rotating mode queue before they (potentially) start work on Mastery and chests for TT and ARAM. And key drops are currently bugged for everything besides (PvP) SR.

So, yeah, Hextech Crafting is essentially nonexistent for Alternate Maps players right now.