If you, or someone else, is still interested in this i have found a way to use SFML with a GTK-Sharp widget.
First you need to disable DoubleBuffering (widget.DoubleBuffered = false), otherwise everything you draw will be overwritten with the widgets background color.
Then you need the gdk handle/xid. Currently the only way is to call a function from a shared library.
For windows this is (tested): "IntPtr gdk_win32_drawable_get_handle(IntPtr window)" in "libgdk-win32-2.0-0.dll"
[DllImport("libgdk-win32-2.0-0.dll")]
static extern IntPtr gdk_win32_drawable_get_handle(IntPtr window);
For linux this is (Tested): "IntPtr gdk_x11_drawable_get_xid(IntPtr window)" in "gdk-x11-2.0"
[DllImport("gdk-x11-2.0")]
static extern IntPtr gdk_x11_drawable_get_xid(IntPtr window);
You have to call that function using widget.GdkWindow.Handle and pass the returned value to the RenderWindow constructor.
Example:
IntPtr tmpHandle = gdk_win32_drawable_get_handle(widget.GdkWindow.Handle);
SFMLApp = new RenderWindow(tmpHandle);
I have tested this with DrawingArea, but it should work on any other widgets too.
[edit]
Tested with Linux: Works
Events work when calling RenderWindow.Dispatch()
[/edit]