Is rubberhose cryptanalysis a sidechannel - formally?
Yes. A human surely is a physical component of the cryptosystem. Even if we ignore the "physical" aspect, we have 


  implementation of a cryptosystem, other than brute force or theoretical weaknesses in the algorithms.


If a key is not secure (ie. in the mind of a human who can be persuaded to divulge it), acquiring this password and bypassing the crypto would be a side-channel attack IMO.