Currently, Open Data Site only honors hidden fields on an ArcGIS Server Map Service when they are first hidden in ArcMap and then published as a Map Service. Along those lines, Open Data Site should also honor the fields hidden when doing so to the map service resource after it has been added to ArcGIS Online. The logic being - honor hidden fields first from Map Service (as it currently does), if additional fields are hidden in the resource and saved (e.g. as a new layer) in ArcGIS Online, also honor the additional hidden fields.
This workflow would help minimize the amount of services that are hosted on our ArcGIS Server, instead of needing separate services specifically for a "view definition" used by our Open Data Portal.
Using Hosted Feature Services (and view definitions) is not necessary a desired alternative either. Since our enterprise data is in SDE databases that is replicated nightly and provides the sources of data for our map services, the alternative of using Hosted Feature Services in our Open Data Portal creates excessive amounts of work for our Analysts to manually refresh these layers whenever the data gets updated (e.g. this process is automatic and is already in place when using our ArcGIS Services vs. going through a manual process for each layer using Hosted feature services).
Please vote up that Open Data Portal should honor ArcGIS Server Map Services hidden fields that are set in ArcGIS Online! arcgis opendata