As I was researching the matter of Partitioned RAID in anticipation of buying a Lacie 2Big (a 2nd one actually), I found this thread and decided I should make some tests to verify some assumptions mentioned earlier on here.
I created a partitioned RAID setup with a Mountain Lion 10.8.5 virtual machine in VMware Fusion, and showed that both OS X and Disk Utility do support partitioned RAID setups as we would normally expect. Whether that translates successfully into a physical drives setup, I would expect so (but hadn't the opportunity to check to-date).
[sorry for the French screenshots instead of English, but you should get the substance of it]
Step 1 : initial setup of partitions. On 2 separate drives, I created 2 partitions on each : 1 partition that's intended for RAID 0 (first partition in what would be the fastest area on a physical drive), and another one that's intended for RAID 1. Same partition sizes on both drives. At this point, Disk Utility does format the partitions with jHFS+ because it won't create blank partitions, although the partitions will get reformatted anyway in the process of setting up the RAIDs.
Step 2 : create the 2 RAID arrays. One is striped RAID 0 (simulated for performance - 2 x 5 GB = 10 GB), the other is mirrorred RAID 1 (for safer archives - 2 x 16 GB = 16 GB).
Step 3 : simulate drive failure. I deleted a virtual disk from the virtual machine, as if the drive had failed and therefore was unavailable anymore to the operating system. I lost the RAID 0 volume as expected, but the RAID 1 volume is still there although in degraded mode. Its contents are unaffected and accessible as I can check in the Finder.
So, regarding LaCie Support's statements that partitioned RAID either isn't supported by OS X and Apple Disk Utility, or would cause 100% data loss in case of single drive failure, I find this inaccurate and instead fully functional. I can't see where the issue would be...