Custom Metadata Relationships and Compound Fields

We don't yet support this. FieldDefinition only includes fields as defined on the object, not individual subfields.

We are thinking about (but haven't yet ranked in our priority list) also allowing relationships to EntityParticle. EntityParticle would include any field or subfield on the object that is supported in a SOQL select clause.

Custom Metadata Types now support relationships to EntityParticles. So you can now reference compound fields in relationships. Take a look at the article for more info - Create a Relationship to a Field Definition or Entity Particle

In Winter '21 release, we are adding support for Geolocation compound field.