I received this answer from Demis Bellot on the Service Stack G+ community.
The official NuGet packages are covered by the Commercial License, whilst the AGPL/FOSS Exception License covers the source code that's on GitHub and is listed in the root directory of SS repos, e.g: https://github.com/ServiceStack/ServiceStack/blob/master/license.txt#L40
All the source code for v4 is on GitHub and Open Source projects can build their own forks and use the test license key at: https://github.com/ServiceStack/ServiceStack.Text/blob/master/tests/ServiceStack.Text.Tests/App.config#L4
The easiest way for OSS projects to get the latest binaries is to build the main ServiceStack.sln in: https://github.com/ServiceStack/ServiceStack/tree/master/src
They can then use the pre-compiled dlls for ServiceStack's sub projects at: https://github.com/ServiceStack/ServiceStack/tree/master/lib Or if preferred you can build the sub projects and use those compiled binaries instead.