Thank you for the info and the magic!. I have been trying various registry entries on my own in the meanwhile. I think I might have stumbled upon the following registry entry…
If I append the following key value to the location of the RUI file (in a writeable location), Rhino is able to display the toolbar automatically.
HKEY_CURRENT_USER\Software\McNeel\Rhinoceros\5.0x64\Scheme: Default\Window Positions\Docking Toolbars\Rui files
HKEY_CURRENT_USER\Software\McNeel\Rhinoceros\5.0\Scheme: Default\Window Positions\Docking Toolbars\Rui files
Can I use this method? Also, while un-installing the plugin, I can delete the entries to reset the toolbars. Is it the right way to de-register the toolbar while un-installing?