I would like to refresh this question, as I am also interested in it.

Anyone has confirmed information? @Bob Reawaruw maybe you did find the answer?

From what I have read, it should work as you described it == only from user UI, while he/she enters the case on a portal.

@Bob Reawaruw I found the below explanation from another user question:

"When you open a work object via the "Find by ID", or "Recent Work item" you are opening the work item itself.

However, when you click on a users work list you are opening the assignment and not the actual work object. The user work list is really a list of assignments and not work objects, so when clicked the actual assignment is opened (of which there could be several on a single work object for different users) rather than the top level work object. This is intended behaviour as the additional call made to OpenDefaults when opening a workobject will not necessarily be applicable to an assignment.

For this reason there is a separate extension point activity for assignments called Work- PerformDefaults when opening an assignment from a worklist."

It might help if you could explain what is the issue you are encountering.  

Perhaps you can run a trace to identify if the reason that your OpenDefaults rule isn't getting called is that Assign-.ProcessAssignment doesn't call any activity that references OpenDefaults.