Timeline doesn't display any icon when isDuration is true. It's justthe way it's implemented right now. You'd need some Javascript coding ifyou want to change its default behavior.
To confirm your research on the list: no, there isn't a built-in
mechanism available to do what you're looking for.
As Timeline stands independently from Exhibit, we have not planned to
update it during the Exhibit 3 project, which wraps up with the calendaryear.