Hello Steven,
Alexandre did not like the needed changes required to add support for this but I figured I would throw it out for debate anyway in case anyone else can make him change his mind (duck). The issue is he does not like needing a single header per-program or per dll. After doing some research it seems to me that even MSVC does not support multiple PCH files per project so I don't know how it could be fixed in gcc to do it Microsoft has not figured it out.
Why don't you create only one single precompiled header file for <windows.h> ? The speedup effect will not that big as in your example. But the code changes are minimal. The <windows.h> include is used nearly everywhere, and it contains a great bunch of declarations. So it can be used for most of all programs and DLLs. Perhaps this change is more easy to sell.
Regards,
martin