I have looked around and I can't find any official definition of sdmx:MetadataProperty. I have found this however: https://code.google.com/archive/p/publishing-statistical-data/wikis/IssueMetadata.wiki
And in there:
sdmx:MetadataProperty a rdfs:Class, owl:Class;
rdfs:label "Metadata property"@en;
rdfs:comment """Class of properties used to provide metadata values in
metadata reports (sdmx:MetadataSet)"""@en;
rdfs:subClassOf sdmx:ComponentProperty;
owl:disjointWith sdmx:MeasureProperty;
owl:disjointWith sdmx:AttributeProperty;
owl:disjointWith sdmx:DimensionProperty;
.
Could you integrate this into this repo? Not sure where would be best, but it should be somewhere. I think sdmx.ttl would make the most sense.