This isn't really advertised anywhere but you can control Caustic using a bluetooth QWERTY keyboard on Android. Obviously you don't get the nice big keys or velocity information, but BT keyboards can be bought for little money and can useful for other purposes while traveling, like typing emails. Some phones (less and less) even have built-in keyboards and those should work too.
The key mappings are the same is in Caustic for Windows:
You can use PgUp/PgDn to change the octave on the preview keyboards. You can use the up/down arrow keys to scroll the rack quickly. You can also use the keys to type song names, presets, etc.
This is Android only as iOS doesn't allow for raw access to input devices (although I think they might now in iOS 7...)