Input component should support hotword detection. See https://www.pivotaltracker.com/story/show/29461159
Is SRGS GARBAGE sufficient here?
I'm going to suggest that for DTMF, GARBAGE is sufficient (and should be implemented in ruby_speech's recognizer), and that for ASR this is a possible solution, while hotword-engine-specific grammars are another. The input component shouldn't care either way.