I haven't ever tried it on Kiwi (thankfully, he's only been sick once, and vet prescribed antibiotics cleared it up), but I have used it on myself. I get really awful ear infections every time I fly (I guess the pressure changes?). Since I have Cronhs disease, prescription antibiotcs mess my stomach up so bad I would never take them except in a life or death situation. I have used colloidal silver for years on the ear infections (a few drops in my ears to clear up the infection) and it works like a charm. I also use the spray form when I get sore throats, and it works really well on those too. I personally can attest to the fact it does indeed work on people, and if the vet has given an ok, I would imagine it could help a bird too.
What I'm more curious about is the reason for the stinky poop. Antibiotics in some humans can cause severe digestive upsets, and I'm wondering if it's possibly messing up your birds digestion/making the problem worse. Antibiotics kill EVERYTHING, including the healthy bacteria in the gut that help digest food and assimilate nutrients. I'm not sure what caused the stinky poop to begin with, but round after round of antibiotics may be making the problem worse since the GI tract isn't being given the chance to repair and regulate itself. Perhaps you should consult with the vet about getting them off the antibiotics altogether for a while to see if that helps. I'm not sure how their diet is, but if it's less than ideal, some simple dietary changes under vet guidance may also help with the smell. Best of luck!