You don’t get to pick and choose where on top a dockbar is docked, programmatically, Rhino’s dockbar manager determines this.
Rhino does remembers the positions of dockbars. Then Rhino opens, the command bar, menu bar, and toolbar return to their previous location. The only place to put your bar, thus, is below the last dockbar, which is the tool bar.
You can, of course, move your dockbar if you need.