If you are arriving in at JNB from LVI at 15.00, and if my memory, from last June serves me right that is the Nationwide flight. Our luggage was through very quickly and as checkin opens 3 hours 45 mins before the flight leaves so you will have a bit of time on your hands.
Last year I went to the Virgin ticket desk, explained I was a UC passenger and could they possibly check me in early? After a call someone appeared and checked me in !!
Its a bit cheeky, but it worked and I will use the same approach on May 3rd when I arrive from WDH a bit earlier than check in opens.
As far as the clubhouse opening times, if you get there early, a certain steward (everyone who has been to the JNB clubhouse knows who he is)!! will happily let you in early and then proceed to try and make sure you 'feel no pain'.
JNB is a lovely clubhouse, with the usual business centre in an upstairs area, an excellent brasserie and some rather palatable South African wines. Apart from grabbing a shower there is not a lot else to do apart from 'pain relief', so do enjoy it! JNB is a sterile airport and therefore I recommend just getting comfortable in the clubhouse.
Enjoy your trip to Zambia, I spent a fantastic 11 days there last year and will return at some point!!!
Mark.