Hi @Oliver_Salway-
We’ve added a feature called File search paths to the latest RhinoWIP (5E199w). I hope this helps your situation. I did some tests with a shared volume between Rhino 5 for Windows and Rhino 5 for Mac and - once the search paths were set properly - the linked blocks and textures worked well.
Please give this is a try and let us know how it goes.
Thanks again for speaking up about this issue,