Hide bookmarker in the timeline, and replace it with an icon.
It is possible to hide the normal bookmark indicators in the Smart Client Settings dialog under Timeline. In here you can also enable “Additional markers” which is plugin provided markers.
Now you can make your own plugin that put markers with your own icons on the timeline wherever there is a bookmark. You can look at the Smart Client Timeline sample for inspiration on how to do the timeline plugin and the Smart Client Bookmark sample for how to work with bookmarks, but it is quite a lot of work just for changing the icons.