I take it from the following statement:
B.1.2. Limitations Although most questions used by debian-installer can be preseeded using this method, there are some notable exceptions. You must (re)partition an entire disk or use available free space on a disk; it is not possible to use existing partitions.
...that the the preseed cannot pass arguments to the advanced partitioner. In this case you would be better off just running the normal install from the LiveCD and when it asks how to install Ubuntu select "other" or "do something else". This will take you to the advanced partitioner where you can select the install paritition for / (root filesystem). If you don't do anything with the other partitions then the installer won't touch them - with the exception of swap. If you don't set up the swap partition I think it will get set up in fstab anyway.
If this is a one-of there's really no need to use a preseed file. Preseeding the installer is really for when you are doing many machines such as an Enterprise roll-out. If this really is an enterprise roll-out then you would need to delete the partition on each machine that the root filesystem is on, so that you can preseed the correct value to use only available space.