Well, you didn't follow my suggestion in Running project with custom properties raises Exception - #4 by wlandgraf?
You are creating a virtual property, thus it's expected it's not displayed in the code completion. It's just a fake property in the object inspector.
Having your own custom scripter class as I suggested not only works but also reduces most of the code to just a few lines. Here is the modified project that works.
CustomProps.zip (6.1 KB)