The night qualification has always been regarded as ab-initio training though I can't find a specific reference for that. The Night qualification is an extension to the licence privileges and is not part of an aircraft rating which is what the CRI is qualified to teach for. It is certainly the CAA's view that the CRI privileges do not include ab-initio night instruction and CRIs have not been taught to teach it.
If the CRI and the student receiving training are both night qualified, there is then nothing to prevent instruction being given at night.
There was an issue some years ago with IR instruction being given at night when the instructor was not night qualified. It was pointed out that the privilege of the Instructor rating with "Night" qualification was to give instruction at night and not specifically for the Night Rating/Qualification and thus was required for any instruction at night.
When the JAA dreamt up the CRI, it was intended to differentiate between a TRI MPA and a TRI for SPA for use by AOC holders to train company pilots and not for the many purposes it is currently being used for. Consequently, initial night instruction was never even considered as a possibility.