I had the same failure on my '82 380SL last spring. It failed while I was making a right turn into the grade school parking lot while taking my granddaughter to school. The left lower rear mount broke. I chained it up and was able to drive it onto a dolly for the tow home. I found a used subframe from a '74 240 that worked ok for me - all I had to do was make the access holes under the engine mounts so I could install the engine mounting bolts. I bought the factory fix bracket kit from the local MB dealer in Portland, OR - $97 dollars for four very small trangular shaped pieces. They also copied the instructions for me, the instructions said to only use a mig welder to weld the brackets on, which is what I did. When I did the job, I jacked up the front of the car about 2 feet or so, and supported the car on jackstands and the engine with a floor jack, using a piece of 2x12 under the pan to spread the load. I used the opportunity to replace all the rubber on the subframe. It is a good time to check the free play in the steering gearbox also.
I am inclined to think that if one of these failures causes an accident MB could be found liable, having incurred the problem earlier in the vehicle's life (The recall affects ONLY USA cast iron block engines, but they all seem to fail sooner or later.)

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C