OK, fixed the DAR, selected NBO. I checked that you can fly from BKK to LHR, then LHR to NBO on a single flight (BA flight #s), and that these flights were less than 24 hours apart (arrive LHR about 5p, depart about 10a). I think that fulfils rule 3.2.1, right? That should be it--done.

Looking at the tool after selecting my 15th segment, BKK-LHR, all the cities in Africa are red triangles (not selectable). When I type in NBO however, it draws in the dotted line, and, so far, no alerts appear on the page of chosen segments. However, the tool does not recognize this as a "finished" itinerary ("itinerary status incomplete"), and prompts to finish, and I get the automatic routing NBO-CPT if I hit the "finish" button. I find it curious that their tool tries to violate their rule. The tool rightly won't allow me to enter LHR-CPT though. Maybe it is pilot error, but I can't see it if so, which is really disconcerting having just yesterday celebrated yet another birthday.