When I come across this, I really see two viable options:
The network share, with folder redirection, which is supported by Microsoft with Outlook 2010+ (see: http://support.microsoft.com/kb/297019/en-us). It probably *works* just as fine with Outlook 2007, just not thoroughly tested/supported as it would be with 2010+. Perhaps you could consider this an opportunity to update Outlook/Office, to relocate those OST files to a network share. Are you using Persona Management?
The other option is to use persistent desktops, but with everything essentially redirected except the local bits that have your OST files. I wish there was an option to have an automated floating pool with persistent data disks, just for user data like this.
I would probably balance the risk of Outlook 2007 with network OSTs and the flexibility of the non-persistent desktops, to decide what's the right option. It might depend on how many users you have in your environment.