I'm in a hi-rise with my B. He gets plenty of exercise, but it helps that my condo layout is very open. He has a good 700sqft+ in the living room/dining room at his disposal while I'm gone during the day, which I think we can all agree is plenty of space for a Basenji on his own. A very welcoming course for a B-500. :D
Best part is, on top of his walks, he gets a daily "sprint" down the hallways on my floor when we come in after a walk. We've had some funny moments when someone decides to open their door, and Kananga feels curious enough to poke his head inside their home. But usually they just witness him flying by.
I've always had him at a healthy weight and he doesn't appear to be frustrated. It's all about how you work with your environment. I know plenty of dog owners that have a yard and a large home that do not give their dogs enough exercise, and it shows by the weight issues.