My idea is to have the ability from the Portal Administration page to set your Portal into a maintenance mode where users can't write to Portal and a notice can be posted for the users. This will allow administrators to be able to take down Portal and work on it and be sure that items are properly stored in Portal, Server and the Data Store. If there is an issue with the Portal, Server or the Data Store the Enterprise won't have any items published as orphaned or have the possibility of corrupting the Portal in some way. To me this would be like the Single User Mode of SQL Server.
I know if you are not using a hosting server you can turn ArcGIS Server to read-only, and you can change the Data Store to read-only, but I haven't see anything with Portal itself.