Although my answer is not authoritative, and TSO C3e does not seem to describe a specific requirement for this, I would think that referring to the slip indicator specified for that aircraft, as a part of it's design, would be a good start.
If someone is trying to "trick" certification by subbing in an indicator with different properties, I would think that the test pilot would catch this pretty quickly. Similarly, the test pilot for a new design will probably not pass a slip indicator which is not affective.
Keep looking though, there may be a prescribed design for this somewhere....