What I've used before is putting multiple items in the same row on a grid.
In my case it was putting a splach screen background graphic up, then injecting a view overtop. If the injected view has an opaque background, it covers the splash.
<Image Source="...your image source" Grid.Row="0" />
<ContentControl prism:RegionManager.RegionName="MainRegion" Grid.Row="0" />
I would think you could put any xaml controls in there just make sure the injected region has something like a grid container with an opaque background to cover what is visible in the parent view.