Comment 1 for bug 109475

The short answer is that partman-auto has no facility for reusing existing partitions yet, and thus you have to use 'clearpart'. This results in the known missing feature documented in https://help.ubuntu.com/6.10/ubuntu/installation-guide/i386/automatic-install.html, which results in your problem:

  "Restrictions of a partition to a particular disk or device, and specifications of the starting or ending cylinder for a partition."

Now. It is possible to work around this, if the system has been entirely pre-partitioned, all the partitions are on one disk, and you happen to know the installer very well. :-) I've attached a translation of your 'part' commands, which you can include in a %pre script. In addition, you will need to remove all your 'part' commands and add 'preseed partman/confirm boolean true'. You will also need to test this to death on a machine you don't care about. I've tested a variation in a VMware instance, but that isn't quite the same. If you need to vary the partitioning scheme in minor ways, I hope the syntax is reasonably clear; it's not entirely unlike partman recipes.

You cannot create any partitions using this workaround. Adding that would probably not be very much simpler than just adding proper support for this to partman-auto.

I hope that helps in the meantime ...