A “used as reference” constraint (Q21528959) is violated when a property is used in a main snak or qualifier, instead of in a reference. It’s analogous to the “used for values only” and “used as qualifier” constraint types.

This should be possible to implement by following the instructions in the README and/or copying Id45d80e7a0. I’m leaving it open for a few days in case a volunteer wants to get their feet wet in the WikibaseQualityConstraints code, otherwise I’ll claim it and implement it (hopefully in time for T176863).