Thanks for the replies.
Another MB contact sent me a note saying that using brake cleaner and spraying inside the throttle body is fine. At least on my 1990 M103.
He said not to remove or change the calibration of the air mass sensor (the round plate which is exposed when you remove the air cleaner.) Also, it is just under the air mass sensor that I see the grime. I guess the TB is further down the passage. I just removed the air cleaner and pushed down on the air mass sensor to see the grime.
By the way, I don't use a K&N filter on my car. He concurs on that point. More problems than it's worth.
I've asked him about whether the car should be running or not when I clean the air flow passage.
Hope this helps others when they are in the same situation.