Well, just quickly looking at the source code, DirectShow uses an IVideoWindow.
In the source code, it sets the owner of the IVideoWindow to the form.
You could, instead, probably use a panel and just change the line:
hr = this.videoWindow.put_Owner(this.Handle);
hr = this.videoWindow.put_Owner(this.panel1.Handle);
Then, you would also have to change the ResizeWindow routine to use the size of the panel, instead of the size of the form's clientSize.