1. Render to to png with transparent background
2. Display png in <img /> and use that as an overlay (same position as map element, but higher zorder)
You can use System.Drawing.Graphics to create the png - possibly using a web handler.
If you are using the System.Windows.Forms.WebBrowser control you can interact with the IWebBrowser2 interface through the WebBrowserBase.ActiveXInstance. WebBrowser is derived from WebBrowserBase.
You should be able to use the handle of the System.Windows.Forms.WebBrowser to enumerate it's children, subclass and listen for mouse events. This is obviously is getting fairly convoluted, and you can hopefully get away with something simpler.
To give you anything more meaningful, I'll have to know more about what you are trying to achieve.
You can use this article as a starting point:
Thumbnailer HTTP Handler[
^]
Regards
Espen Harlinn