Just to make it absolutely clear: I really only had a quick look and just thought creating a thread would be a good idea so that we don't do the same stuff over and over again. A lot of what I'm writing are just first guesses, nothing more.
It seems that Carma on Android is moddable. The data itself is not in the APK (which mostly contains video and music). Instead it's in an OBB file at Android/obb/com.stainlessgames.carmageddon.
I don't really know anything about that Opaque Binary Blobs as Google calls them. From the documentation it looks like they are supposed to be mountable partition images with optional encryption. But looking at the various OBB files on my sd card it looks more like people are using the OBB delivery mechanism to store whatever they want on sd. The carma obb is definitely not encrypted, that much can be verified with a quick search for the WAV magic word RIFF, but Google's JOBB tool still can't mount it. Looking at it in a hex editor, it doesn't look like any partition format I know. It does contain a magic word at the very end "[version]..1320", which I don't recognize either. The beginning could be a magic word or not, but I don't recognize it either. It's just 0x01000000 three times.
After that it says "DATA_ANDROID.WAD(" , which sounds like the whole OBB file is just a dummy container for this homemade WAD format. WAD is not really a specific format, today it's really just an alias for "simple uncompressed file table" (WAD apparently stands for "Where's All the Data?" according to Wikipedia... I honestly didn't know that).
After that there's a long list of file names and after that there's a table of what looks like 64bit numbers, probably offsets.
That's all I've gathered so far. It looks promising, but of course there could be checksums or other mechanisms for preventing modifications.