I've found answers to my XAML menu throbbing questions! Using RoutedEvents in ControlTemplates (and DataTemplates!) lets me start and stop timelines for animations. I can place these triggers inside styles, and it turns out TargetName isn't always required. A cool discovery: storyboards inside ControlTemplates/DataTemplates can be accessed by any control matching the style's TargetType. My wobbly menu example demonstrates these techniques.
I've encountered a crash in XAMLPad (May CTP) when animating the width of a MenuItem. Specifically, if I remove the "From" attribute in a DoubleAnimation within a SetterTimeline, XAMLPad crashes. I expected the animation to start from the MenuItem's default width. Is this the correct way to use DoubleAnimation without specifying "From"? The crash occurs in a specific context: the Timeline is defined within a Style applied to a HeaderTemplate's DataTemplate element for a custom-drawn control. See the linked XAML for a complete example.