Similar situation used to occur for QF and SAA at LHR, with wide-body aircraft towed to maintenance aprons for a long layover during the day.
QF did try to make use of the B744 in the past by operating LHR-MAN-LHR, but it probably wasn't an economic use of such an aircraft. Even harder to make a success with an A380 which is even more operationally limited.
BA could potentially utilise the long day stop in JNB for a short sector to WDH or MRU and utilise the route licences of Oneworld partner carrier Comair. However, the larger wide-body aircraft may be just too large to operate these sectors profitably with the need for additional crews adding to the costs.