That's a good question, SP!

So basically I had thought this situation through in advance. I flew LHRJNB on Tuesday (on Virgin!) and took a case with my wife and kids' stuff, in order to enable them to travel hand baggage only - which they were doing. I realised that (i) BA very likely would not be able to check the case through to Mauritius and (ii) that my family would not be able to enter the baggage reclaim hall at JNB, after immigration.

There was a relevant (minor) development after my long initial post. Seems some BA staff took pity on my wife and kids (must have been a pathetic scene - probably you could hear the wailing from one corner of T5 to the other at that time of night), and tried to help. One thing they have done (of currently uncertain benefit - let's see) is waitlist the three of them on the LGWMRU flight tomorrow night. But as I was talking to the agent (different one from earlier), who essentially took the line that "what my colleagues did was right but I am going to try to help you anyway", he said words to the effect of "well of course you couldn't do what you planned to, because we can only check your bags as far as JNB and you'd have to clear immigration to get them".

Er, HBO. Something BA promotes, even on longhaul flights.