Way too risky to do this.
Naspers already dropped from R2k+ to R1.8k. If you buy now, it may seem like a bargain, but potentially, Naspers can, and probably will still drop down to R1.7k, R1.6k or eveen R1.5k. In the event of junk status credit rating, it may dip below the R1k mark at the end of the year. Then it will not be such a bargain anymore.
You can take a look at what happened to Brazil's stock market last year. Only one company declared them junk. But, before that, starting January 2015, international investors started withdrawing (much the same happening on the JSE now). By August investors already withdrew around $500 billion from Brazil stocks. Most of their stocks already dropped between 30-60% in value by then. When they were declared junk, the stock market fell. Investors who still had money invested lost about 30% in one day and they could no longer find buyers.
The problem after such incident is that you do not know where the value of your stocks will end up. The next problem is that the value that remain may actually stay that low for the next 10-20 years. The value may not even return to where it was in our lifetime.
So, knowing a country is about to be declared junk is a huge risk to have any investments left.