Interesting question.
There are many standard and metadata profile implementations (ie INSPIRE in Europe has different members's states impementaions).
I made the same question ant they said that actually you can only add your custom metadata profile in Geoportalserver but it's not possible in agol- enterprise.
I thnk that this is the BIG GAP and ArcGIS enterprise should integrate a new-geoportal server or add customization functionalities, at leats to local/national distributors so they could implement /offer a solution to thei re local customers.
