I downloaded the MB part number charts for the keyblanks and ignition tumbler parts and the W124 obviously uses different parts that the W201, the W123 is also different.
Thanks for the pics Haasman! Great help.
The dimensions (thickness, width and length) of the key blade is the only factor that would make the flip key remote feasible, since the flip hinge has a limited amount of play (on the ML keyholder, the W201 key has to be ground down in thickness and width to fit the hinge) and the length of the groove along the side of the remote will dictate how long the keyblade can be. I'm still waiting for confirmation from Germany.