Bookmarks isn't particularly entangled in the old UI, I had it working in my attempt at a docks based UI.
I don't really see it as different from the other plugins listed
in #1154, it has state persistence, but I don't think that's a GUI tied issue.
That said, the old GUI worked by letting plugins register as things that could provide a widget for the GUI to manage. It also handled persistence in that process. In that context bookmarks.py would work equally well in a new GUI that registers a list of things that can provide widgets to manage. If the new model is for each plugin to understand the GUI and manage itself within it, then there might be more work to be done.
Cheers -Terry