"bmi are telling me that LH do not just restrict availability at this this time but have a complete blackout even for *A Gold. "

Obviously some seats are available to LH Miles and More members.

I would try the dates posted on the LH forum (5, 7 and 8 of January JNB-MUC-FRA) and see if you get a positive. It is a busy time of year and they may have very few reward seats for this route. Going via Munich will help since that is a fairly new route that people are largely unaware of.