Eventually people find out that widgets can, and therefore inevitably will, intercept right clicks and make it really hard to get at the applet's own contextual actions. Right, Will?
There's nothing Plasma can really do about that. We could try and intercept every right click and show a menu, but what if a widget actually uses right clicks for something different in a defensible use case? It would also be a bit hacky to make that work perfectly, though doable.
So, we have a situation where controlling right clicks is a problem. On the desktop, we offer widget handles and the desktop toolbox so we can guarantee levels of service regardless of what plugins do.
On the panel, we also use the toolbox button to a similar end. If you click on a panel's toolbox and bring up the configuration interface, you can then right click on any applet ... and get the applet's own context menu!
So don't hunt for the one pixel in between taskbar buttons or around the system tray, just click the cashew and then right click the applet. Voila.