I would NEVER terminate an unused "hot" conductor on a ground/neutral bar. There should not be a chance of it becoming energized if both ends are properly capped off.
Even though it may not be acting as a ground or neutral I still would call that a violation.

What I have seen in residential applications is "3-wire" cable being used for isolated ground circuits. This is basically a useless effort in a residential setting with NM cable anyway, besides the obvious code violation of using a red as a grounding conductor.