Since BorderRadius is missing from the Style definition/metadata, there are a few options:
Workaround:
Script.SetField(div.Style, "borderRadius", "...");
Fix the piece that is missing:
Update the code in https://github.com/nikhilk/scriptsharp/blob/cc/src/Libraries/Web/Html/Style.cs and submit a pull request.
Or at the very least, log an issue.