How can I get the file version for a .sln, if possible?

[kinook]

A .sln file doesn't have a version number; each individual project within the solution usually does, and you can use the Get VS.NET Version action to retrieve a project's version(s) into temporary macro(s). If you're using a Visual Studio setup project to create the .msi, you can retrieve the product version from the corresponding .vdproj file.
http://www.visualbuild.com/Manual/vs_netprojver.htm

The attached sample also demonstrates retrieving the product version from a .msi file and the file version of a .dll file into temporary macros and incrementing the version.