This capability is not possible in Portal for ArcGIS. May I suggest a different approach: instead of pre-loading favorite maps/items to all of your Portal members' profiles, why not leverage the Gallery option in Portal?
Take all of the items that you want to promote as favorites and place them in a group. Then assign that group to the "My Organization's Feature Content" tab on the Gallery page.
To enable: My Organization > Edit Settings > Gallery > select the group.
Then simply direct your Portal members to go to the "My Organization's Feature Content" tab on the Gallery page.
Hope this helps,
It is interesting how you can come across a really old post and still find relevance. To answer Derek's question, the answer is very simple today. Mobile apps, ArcGIS Earth, several have a category for favorites. It would be easier to direct users to this generally accepted functional area than force them to log in to Portal, and favorite the item first. Some of our users never use the web version of Portal.