mmhh cant say too much about the details, but grasshopper is in fact developed in .NET, and still runs in Macs based ion Mono project. .NET is definitively cross platform, at least that said the advertisement ; )
http://www.mono-project.com/
It seems also that this dll problem is somehow solved by mono project
http://www.mono-project.com/docs/advanced/pinvoke/dllmap/
About the not working plugins of grasshopper on mac, not sure about the reason as long as I am windows user... Maybe the reason is that Rhinocommon for Mac is still in WIP. Just guessing...
Anyway, as I said, i´am not an expert programmer by no means, so i´am not seeing the whole picture, for sure...