Why do you want do it? What kind of aim do you want solve? What are you mean about ""Participants" selected by default"? You mean about default status or default transaction? Can you explain what you want do?
When a user changes the state of a task, he has to select a "Participant" in window "Operation assignee". We need the participant be selected by default, without action of the user.
Not. you can't do it. You can change status of task only use transaction. I advice you create new operation looks like operation 'Note' in default configuration.