Menu item as in "touch upper portion of the screen" have the menue interface show up, including the menu button in the upper right. Click on the menue button. Have multiple menue Items show up (Bookmark Page, ...). Could you add one item for the click event ("Change progress indicator") there?
The aim is to allow changing of the indicator without leaving the book itself.
If it isnt possible without the help of other modules, or it goes against your design principle for jbpatch feel free to say no.