Clearing an arbitrary bit actually is supported (as mentioned in the article: "you can set, clear, or toggle any bit with one instruction").

The specific details require you to dig a bit past explaining just the immediate encoding, but in the clearing case there's a dedicated instruction for clearing the bit specified by the immediate:

  BIC - Bit Clear (immediate) performs a bitwise AND
  of a register value and the complement of an immediate
  value, and writes the result to the destination register.

As I mentioned in my other post, zero-rotation encodings are gamed out as well (to allow byte repetition).