Can you always use the Chassis as a HVAC C-wire contact point?
It is quite common for the C terminal of HVAC transformers to be bonded to the chassis. However, using the chassis as a C terminal is not a common technique, and should only be used in very rare circumstances. 

I'm not sure why I would have suggested that, as it's not a "proper" wiring technique.  If you can link to the question where I said that, it might shed some light on why I said it.