I know exactly what you are talking about. And I did it once! It worked for about a day, and then I let my brother borrow my car, and he turned the key too hard, and it broke.
Ok, well I had a flip key to a 1997 C280. I don't recall exactly how it came into my possession, but it was my friend who I hadn't seen in years (I actually saw her yesterday for the first time in 2 years! Sorry for her... she just got a new Tahoe). Anyway, I was able to get the key itself out of the silver hinge peice, I remember it taking a long time, and some random tools though.
Now I knew that I just needed to get one of my extra car keys, and get it flat like the peice that had come out.
Basically, I had the regular key (round black) for older Mercedes (my 95 C280 has old style key), and cut off off the black rubbery peice. Then I took it to the hardware store and had them cut off the edges to make it straight; then I had to have them make the end (other end of the key KEY (where the rubber used to be) thinner to fit into the flip key's hinge.
So now I had the flip key with no key, and an old key of mine that was cut to fit. The hardware store man made the end (when he was making it thinner) of the key a little too thin. But I figured that was OK, because I would be using some JBWeld to 'glue' it in to the flip key's hinge.
I let it dry, and it was good to go...... for about 30 hours, or until I let my brother borrow it. That's ok though... because it obviously wasn't strong enough to last then.
I am sorry... it was difficult to describe the process and I don't think I was too clear. But I hope you get the idea.
I know this is very possible, because the physical key of my 95 and 97, for example are the exact same. It's possible, but I don't think MB would make it possible.