SVFR is never available in IMC. If your licence privileges preclude compliance with mandatory IFR (for example, a PPL without IMC Rating in a Class A control zone), then you may fly SVFR if the visual conditions are within the limits specified by the controlling authority.
The 5km (VFR) versus 10Km (SVFR) visibility requirement for unrated PPL holders flying SVFR in Class D control zones is indeed odd.
The answer?
When flying as wingman in close formation in cloud. You are flying visually using the formation reference points of the other a/c even though you are in IMC. So you're not flying by sole reference to instruments......