So I dug into the command line options, and then found this thread: client command line option -wallet= in addition to -datadir=?
Basically there's no easy way to run the standard Bitcoin.app (ie the Mac app) with the data directory in a different location, and even on the command line, there's no way to specify a different location for the wallet.
Does anyone see this as an issue? As it stands, running the app on a Mac with FileVault basically renders the machine unusable. I don't know what it's like on an SSD, but I suspect it could drastically shorten the life of the drive.
I'm trying to imagine getting my mum to use Bitcoin, and having to explain why her hard drive is constantly clicking and her machine slows down when it's open. I'm trying not to imagine telling her how to run the app from the command line with a wallet symlinked to a different location.