does anyone have any experience with tiny, tiny func_details acting in this manner (the size of the railings are 1/3hu)?
It's not just standard wisdom it's a hard limit of the source engine. It can't do brushes that are below 1hu in any dimension. Those are called Microbrushes and will never work. If you NEED something to be smaller than 1hu in any dimension, you have to use a model.