Is your iDisk a little poky and just not quite big enough?
Build yourself an iDisk locally on your server! Use Backup.app to back up your Mac systems to your own file server. See the detailed Solo .Mac article here.
Ed. Note: While the article walks you through doing this on a FreeBSD box, the methodology will help you to do this on your own server. The biggest sticking point is the DNS, since you’ll need to co-opt the iDisk DNS records. The article has you just stomp on the entire apple.com domain, which is probably a bit ham-fisted. Instead just create a host record in /etc/hosts for the specific address that you want.