Thanks to a few sentences worth of help from robert ginda on #mozilla, and tons of walking thru mozilla breakpoints to see how XUL wires up command handlers and executes them, clicking a button now results in the following output:

2005-07-20 20:30:14.653 layout[22453] Help! I've been clicked!
 Button::ButtonPressed success
 Button::ButtonPressed. return is a number
 Button::ButtonPressed. return is an int
 Button::ButtonPressed. return is a primitive
 Button::ButtonPressed return is 17

The JS is:

function Button1Click()
{
    return 17;
}

So, what's the big deal? My layout engine is calling onclick handlers for button elements, that's what!

So, as a next step, I need to see if I can create an object that interfaces to C++ and provides logging services. Dumping output to stderr should be good enough. That way, I can write something like this:

function Button1Click()
{
    logger.print("Button1Click: value is %d\n", 17);
    return 17;
}

Fun stuff :-)