To clarify, if I am doing this widget based on a custom web part, yes, I can easily call Guid.NewGuid() and generate a unique ID. This is based on the basic Static HTML widget from Kentico, and I'm looking for a solution directly in the widget, without customizing the web part.
@Brenden Sorry, due to the above (which I admit I failed to be clear on in my original question) I needed the solution to be in the widget itself or its (Text/K#) container.
@Peter Referencing {% InstanceGUID %} actually worked perfectly, thank you!