I have the exact same question and goal (SSD for system drive and HDD for storage) and have read the same documents after googling around. My PC is a DC7800 SFF, though.
I currently have XP on an SSD and 500 GB of storage on an HDD. IDE emulation is on. Planning to install Windows 7 on the SSD and take advantage of AHCI.
Have you taken the plunge yet, and simply tried switching to RAID in BIOS and see how it goes? I'm hesitating to do just that, so I'm hoping maybe you already tried and could tell me if it worked?
My goal for now is to have both XP and 7 on the SSD, because it's a PC I've built for my parents and I think my dad will want to stick to XP despite end of support. I've read guides on how to enable AHCI in XP after installation.
I do have an unused SSD so I could safely just swap out the one with WinXP and try to install Win7 on the new one with BIOS set to RAID and see if it can still use all three devices (SSD, HDD, ODD) in AHCI without forcing a RAID. If you haven't tried it already then I will.