That's not available in Fireworks. Good idea, though.
Alternatively, you could create a new layer to put them on. Then you can turn the visibility of the layer off or on to see the items you have stored.
You can name the layer "Resources" or "Staging," as a reminder of what the content is.
Another option is to create a separate page that is for resources. FW pages can be sized independently from each other, so you could have a single "assets" page that is much larger in dimension than the rest of your design.