I think you need to explain a bit more about what you're asking for. The built-in screens (springboard, grid, etc) are implemented in the firmware -- there is no Brightscript associated with them.
BTW, some time ago I started a little side project to reimplement all the standard screens in Brightscript using roScreen. The idea is it would then be possible to make arbitrary customizations of those screens. I got some of the infrastructure done and a few of the simple screens, but lost interest in it. I wonder if anyone would be interested in picking this up? Might be a good community project, since the work can probably be divided up easily. I want to emphasize that this is just a personal project; it is not authorized nor supported by Roku.
--Mark