RCP Command with object argument

Question

I want to call an RCP command in code, like this:

IWorkbenchWindow window = PlatformUI.getWorkbench().getActiveWorkbenchWindow();
    IHandlerService handlerService = (IHandlerService)window.getService(IHandlerService.class);
    handlerService.executeCommand(cmdID, null);

With considerably more code, I can call the command with a string argument by assembling a Parameterization object then building a ParameterizedCommand and so forth but Paramaterization only allows for string values, and can't be subclassed.

What I really want to do is call the command with an object as a parameter. How can I do this?