You can use the "/Save" and "/Load" options to store and load source information for a static library, respectively. Using these options allows you to store information for your library and then later import it when indexing a project that links against your library.
When indexing your library solution, you specify the "/Save" flag with a directory in which to store index information about the library's source files. For example (assuming you are using Subversion for source control),
ssindex.cmd /System=SVN /Save=c:\source\libproj\srcinfo /Source=c:\source\libproj /Symbols=c:\source\libproj\Release\*.pdb
When later indexing your project that includes your library, you specify the "/Load" flag with the directory containing the library's source file information. For example,
ssindex.cmd /System=SVN /Load=c:\source\libproj\srcinfo /Source=c:\source\binproj /Symbols=c:\source\binproj\Release\*.pdb
There are two potential issues that may affect your ability to use this technique. First, it appears that some source control providers may not support saving and loading source control information. I know that the Subversion provider does and it looks like the SourceSafe provider does, but I haven't checked any others.
Second, this technique appears to only work for one external static library out-of-the-box. There does not seem to be a way to load information from multiple directories and the scripts currently overwrite the contents of the directory each time you use the "/Save" option. You could probably edit the source control provider module to append to the files in the save directory rather than overwrite them, but I have not tried it.
Also, note as you mentioned above that you only need to do this if your library is being built as part of a separate solution. If the static library is part of the solution you are indexing, its source files will be included if they are in the path specified by the "/Source" option.