With this identified, how should the top panel be treated? Visually, it needs a differentiation from the normal clickable state.
On the other hand, we can also consider changing the panel's behavior instead of the visual treatment, like how it's done on GNOME Shell while on Overview
http://img6.imagebanana.com/img/8yunbmeb/Workspace2_008.png . Let's keep in mind that Lens and Overview may not be apples to apples in this case, though. Does it really make sense to make the panel inactive while a Lens is active? It may, but this will also depend on future plans for Unity.