You do realize you still have to download them twice right? The Mac and Windows versions are distinctly separate from each other as installations. Maybe it was just the way it was worded as I am thinking you probably already know that. You will need to explicitly tell Steam you want the Mac games on the hard drive as well since otherwise it will install them on the SSD where OS X system folders are. Maybe you already know that as well.
In Windows, I installed the Steam client directly on the optibay drive and downloaded a small game that's compatible with both operating systems (Peggle Deluxe) so I could ensure I have the complete folder hierarchy established. Then in OS X, I used a program called Source Finagler to create a symbolic link to the SteamApps directory on the optibay drive.
Then I downloaded Portal 2 in OS X. It runs fine. It even syncs my Steam Cloud saves between operating systems. I have to run the game at a different resolution in Windows, but I resolved this by adding command line arguments to the OS X install so it boots up in my desired resolution while staying at the one I want in Windows.
Now, this isn't going to work for every game. Actually, it might only work for Valve games now that I think about it. I'll have to experiment with something like Bioshock Infinite or Tomb Raider, but I don't have time at the moment to dig deeper.
Still doesn't answer the initial question I asked about moving MAS games to the storage drive. Heck, I might download one of the smaller games I have (Doom 3) and try it myself.