I see. What you omitted in manual partitioning is setting up an corresponding entry in the Protective MBR of the drive. When Boot Camp sets up a partition for Windows, it creates an entry in the GPT *and* an entry in the protective MBR (a "hybrid" partition table, necessary because Windows XP does not boot from GPT). It's the MBR that Fusion 5 reads when scanning for boot camp partitions.
If you were to (manually) build the proper MBR entry on your disk, Fusion should be able to run your Boot Camp partition as a VM. Be sure to mark the partition type as 0x07 (NTFS/HPFS).