VB6 and VS2003 building

Not that I know of, because MSBuild is intended to be a generic build tool, analogous to nmake. (It just happens that we are currently only shipping targets and tasks for certain project types.)

However, you will have a lot of work ahead of you if you have to define all the necessary build rules and to parse the solutions. (If you just have to write a task to wrap an existing command line build too, well that would be easy.)

It will be an endeavor, and you may want to hook up with some other people with the same interest, and collaborate.