I'm sure that Queensland Rescue have operated their nightsun-equipped 412s IFR for years, ie plan and fly to an area of operations IFR, then if conditions allow, descend visually using the nightsun.
It would seem improbable that they would have simply decided to ignore a prohibition to fly IFR with the equipment installed, so I assume they have some kind of approval to do it.
As far as the effects of the nightsun on IFR operations, the thing that springs to mind that might affect other systems is the big current draw for such a powerful light - ie big current through nightsun circuits creating induced currents in other instrument systems that affect their accuracy, obviously not a player when the light's off.