Well, it does seem to conflict with what allergy-oriented theorists have suggested. Every medical professional (including my allergist) that I've heard talk about niacin in relation to POIS seems to believe that it works because it's causing the depletion of histamine stores. The flush is what causes this depletion. Unless niacinimide also causes such a depletion by some other means (which is entirely possible), it's effectiveness indicates that those individuals are at least partially wrong.
In any case, niacinimide doesn't work as well for POIS prevention as niacin. In fact, I wouldn't even recommend using it unless you're already under POIS and need immediate relief. But when I have used it for prevention, I've taken 400mg around an hour before hand. It does seem to diminish the symptoms somewhat. And it seems to do so even when one isn't on an empty stomach. Still, it doesn't even come close to niacin.
If using it for relief, I'd say: 1) don't use it before Day 2; and 2) try taking 300-400mg a day. It helps me a little when i'm fully under.