I’d like to make use of the FOSS exception in an open source project I just started. (I suppose the NuGet packages still can’t be used with the FOSS license key.)
Is there any easy way of building the .NET Core versions of the ServiceStack projects, either as packages or as DLLs?
You would need to build the solution in each repo that you want to use, if preferred (and when exists) you can use the pre-built .dlls in /lib folder. See this answer for details.
Hmm, I’ll likely need an earlier version of the tooling, as you still use project.json/*.xproj. I like challenges. 
I ended up doing it like this: https://github.com/dabide/FloridSword/blob/master/updatelib.sh
Not perfect, but suits my needs. I replaced the version number with a very high one to avoid finding updates on NuGet. (It seems that DLL references are also possible in the latest MSBuild based versions of .NET Core.)