I really can't believe that the pin material makes THAT much difference. Any pin that fits properly and does it's job should be OK. I could see that, maybe, a badly fitting pin or a soft pin that allows the string to move might be a bad thing but the pin's job is to stop the string from slipping out of it's groove and if it does that properly then there should be no noticable difference between materials.

The vibration should be transmitted mainly through the string resting on the saddle with a minor amount transmitted through the ball where it is seated on the underside of the bridge.

Regarding brass pins sounding brighter, I would suspect that brass, being a harder material, might make it easier to ensure that the string is fitted correctly because it would be less prone to binding on the string and stopping the end from being seated correctly. In that respect the harder the the material the better.

Bert.