Until now I've been using a toolbar to let the user load / activate its plugin...
Toolbar are end-user created UI widgets that run commands or command macros. If you run a command (from the command line, a menu, or a toolbar) and the command resides in a plug-in that is not loaded, Rhino will automatically load the plug-in (if it is registered) and then run the command.
Like I said, the Rhino Installer Engine does not automatically register any .RUI files, included in your .RUI, for automatic loading by Rhino. It simply installed them (e.g. copies) them into a location that can be easily found by a plug-in that wants to load them. User can also load .RUI files themselves, using the Toolbar command, if they know the location.
Your plug-in can load and unload toolbars. If you are using C++, see the
CRhinoUiFile class. If you are using .NET, see the
Rhino.UI.ToolbarFileCollection class and associated classes.
Also, regarding toolbars created and modified within Rhino's UI, if you give the .RUI file the exact same name as the plug-in .RHP file and install it in the folder containing the RHP file, Rhino will automatically stage it in a writable location and open it at startup. Note, staging is required because each user on a system get their own version of toolbar files so they can edit them.
Hope this helps.