Hi Koen. In the past when I wanted to see broadcast safe margins inside C4D, I would make a PNG in Photoshop with the lines I needed. 16:9 of course. Since the Foreground Object isn't supported, I make a plane that is 16:9 in proportion and drop this material on it. The important part for my case was to make this plane a Child of the Camera and adjust the plane's position so it fits your field of view perfectly. Now, no matter where I move the camera, I can always see my broadcast lines. I also add a Compositing Tag and turn OFF the Seen by Camera.
It would help if you described your situation with more details or posted a screen shot of you scene. What do you want to put on the Foreground Object? Some of what I posted above may apply to you, but don't know enough details.