This is possible and it is shown by the VideoViewerQuality sample, please see this –
I see that I can add shapes, but not that I can add a panel or usercontrol with buttons this transparent background, or am I missing something.
In a Smart Client plugin you have VideoOS.Platform.Client.ImageViewerAddOn.ActiveElementsOverlayAdd
This samples shows its usage: https://doc.developer.milestonesys.com/html/index.html?base=samples/activeelementsoverlay_sample.html&tree=tree_1.html
In the standalone scenario using ImageViewerWfpControl or ImageViewerControl you do not have this and will have to resort to using VideoOS.Platform.Client.ImageViewerWpfControl.ShapesOverlayAdd
There are elements you can use in the Smart Client environment but not in standalone. I guess you can do transparent shapes but perhaps not more advanced elements.