Resource scripts loaded from .rgs files are used with ATL CComModule::UpdateRegistryFromResource(). It's quite common to see samples of such files on the net but I've never seen a complete description on what constructs are allowed in .rgs files. Is there such a document?
There's some MSDN documentation, but it appears to be pretty unfocused.
The book ATL Internals talks at some length about custom replacements.
Other than that, I haven't seen much...