To answer your question, alex, the only reason I am interested in doing this is sheer convenience. I could just as easily use the seat belt, but my wife and I make every effort to drive my company car (A BadA** 2007 Ford Taurus with a tape deck, ask me about it) as much as possible because it is much cheaper to operate for me (5.5C/mi for personal use). We switch the seat back and forth almost daily it is much easier to install and remove the car seat with the LATCH system than with the belt. No other reason. If it were as easy as unbolting an old part and bolting in a modern part (post 2002) I would.
I stopped by the dealership today but they didn't have any post-2002 wagons on the lot. Also, I have called the parts departments of 3 different dealerships all with the same result- the parts guys show that the parts diagram for newer cars isn't clear, so they don't know how many mounting holes the new anchors have. Phil at fastlane also referred me to the dealer because he can't support parts for airbags or seatbelts.
I probably would have given up on this one already if I didn't have to answer to "The Boss" with why I can diagnose and fix (or pay to have fixed) almost anything else with older mercedes cars except the "ONE" thing that she wants...Tough argument.
Sorry for the rant.