I don't think that is easily possible at the moment... It is of course possible to mimmick how it works in c++ with pyside (find the toolbar and add an actiongroup to it), but, if you plan to add python commands to the existing Drawing WB, I think I would do the same as in PartDesign: simply create your python commands in different files, and import them in InitGui.py. Then, we can modify the C++ workbench to add these commands to a QActionGroup.
If you don't want to create full FreeCAD commands, but just create some qactions, then I would create an additional, pure pyside toolbar, same as I did for the Draft snapping tools (that toolbar is created and managed in Draft/DraftSnap.py, line 1162 show() and line 1068 makeSnapToolBar(). There you can do all the stuff you want, independently of the other toolbars, and not use FreeCAD's command registering stuff.