Good news and Bad news.  The good news is that you've already found most of
your answers.  The bad news is that the IDE won't help you much, if any, to
do this.  You are correct that you're not using .rc files in your project.
What is happening right now is that the compiler is generating the
equivalent of an .rc file for you and using that to set your output's file
version info.  The complier generates this based on several of the assembly
attributes.  Unfortunately there is no attribute that correlates with the
SpecialBuild field.  So if you really want to set it, you'll have to
manually create the .rc file (the VC project system has a great .rc file
editor which includes a rather nice file version info editor).  Hopefully
you're using 2003 or somebody else's build system because you'll need some
custom build rules.  As a pre-build step you need to run rc.exe to compile
the .rc file to a .res file.  Then you need to find some way to pass this
file to the C# compiler's /win32res command-line option (I can't remember
off the top of my head if this is even possible).