Suppertime, this is just my 2-cent but I think the fact that you do not ejaculate might alter the relevance of your results. It is the ejaculation, not the orgasm, that causes POIS (that is why I think the name PEIS would be more accurate). I had several successful, ejaculation-free Os, and never felt any bad symptoms afterwards.
If your masturbation or sex does not lead to an ejaculation, I believe you shouldn't need the niacin or anything else to avoid POIS. If you feel you need it regardless, then it is probably because you do ejaculate a tiny bit, or you emit semen the moment you go peeing. If your post-masturbation or post-intercourse pee is a bit thicker and it makes bubbles that won't pop as it hits the water, then you're probably emitting semen, mixed with your pee (and you are likely to feel the effects of POIS). That is, at least, my own experience. I call it "false ejaculation": no semen comes out (because I squizz my PC muscle) but I can tell it still has gone "too far" (I squizzed too late). In that case I will pee it as soon as I go to the restroom... Then I am mad because I messed up my O AND I know POIS is about to kick in!
I also had a question for everyone (sorry if this has already been answered somewhere else in the forums): is niacinamide as effective as niacin? And what about "flush-free" niacin? Anyone as tried them all to compare the effect?
Thanks!