A “used as reference” constraint ([Q21528959](http://www.wikidata.org/entity/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](https://github.com/wikimedia/mediawiki-extensions-WikibaseQualityConstraints/blob/7f90094d56545e7624157265b3f3bdb9ccf33fb0/README.md#adding-a-new-constraint-type) and/or copying [Id45d80e7a0](https://gerrit.wikimedia.org/r/#/c/381005/). 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).