Wow.. where've you been?
The python card update script that was submitted did the hard part of adding those lines in. I made the modifications to the non-draft booster pack generators to build their lists from the SVar's instead of the rarity.txt files. Mythic currently just maps to rare, but it's parsed for explicitly. I have some code that may enable Mythic to operate on its own, but the current Mythic:Rare proportion is probably not big enough to approach the 1:8 distribution, without constant repetition of the Mythics we have. Without handling them specifically, they already show up in "surprise" quantities, which is good enough for now.
Since then, I've gone on to reduce cards.txt into individual card files, which will enable us to implement additional meta data for cards, without Forge counting them as keywords initially.
The picture URL and rarity will also be combined as a new piece of meta data - "Set". Cards printed in one set may have different rarity than other,
Serra Angel, for example. Eventually we can build set-specific boosters (at that point Mythic will become fully functional). Eventually we could enforce Standard or Extended... either real-time or "choose your own standard".
I even plan to revise the spell and ability keywords, to not be keywords, but directly interpreted. Keywords like Flying shall remain Keywords though.