Sure.
I messed about in terminal but couldn't get it to work so I downloaded this tiny service app:
1 - Download SymbolicLinker: http://seiryu.home.comcast.net/~seiryu/symboliclinker.html
2 - Mount the dmg file and copy the .services file into your (Mac HD)/Library/Services (if its not there add it)
3 - Open the SymbolicLinker.services file from the folder you just copied it into
4 - Navigate to your server or dropbox or second hard drive - wherever you store your materials collection
5 - right click on your materials folder and at the bottom there will be a new option saying "create symbolic link"
6 - now select the new symbolic link it just made and copy it
7 - Navigate into your sketchup folders ~/Library/Application Support/SketchUp 2015/SketchUp/
if you can't see it use; finder->go->go to folder and copy that in and press enter
8 - delete the sketchup materials folder (OR RENAME IF YOU WANT TO KEEP THE DEFAULT MATERIALS) - I've already collected these into my custom folder anyway so I just deleted them.
9 - paste (your symbolic link) and rename to "Materials"
All Done!
Can do the same for style and components, you can also use these instructions to store other folders on a second drive if you use an SSD.