Is there any easy way to figure out which cities are available on the way from X to Y. For example, if I want to get from JNB to somewhere in Europe and visit another city or two in Africa on the way, how would I figure out what's possible? Is it just a matter of poring over schedules, or is there something better?

My current thoughts for a DONE4 are
JFK-SFO-LAX-SYD-AKL-SYD-JNB-LHR-DME,SVO-BUD-FCO-JFK.