My project on the PC makes extensive use of System.IO.Packaging. This is supported in Mono's WindowsBase library, but this does not appear to be part of the standard Xamarin packages? Anyway, I copied that dll to my Mac and imported it into my References. Although the DLL appears and there do not appear to be any errors related to it, I still get the error "The type of namespace name 'Packaging' does not exist in the namespace 'System.IO'.
This appears to be the last remaining hurdle to getting my code to run properly, so you might imagine my desire to fix this.