I have created a DIY Fusion drive on my late 2011 Macbook Pro using a 256GB SSD and the original 750GB 7200rpm hard disk. I devoted 100GB of the SSD for Windows 8. I have also done it with a 512GB SSD with part again devoted to Windows.
It worked well, until I had to reinstall Windows, and couldn't do it without re-installing the optical disk. I also had to re-install the optical disk in order to perform a firmware update to the SSD. This was made more difficult because I couldn't simply remove the hard disk and boot from the SSD while the optical disk was installed.
However, in retrospect, I might have been able to put the hard disk portion of the Fusion drive in a Thunderbolt enclosure and still boot, or I probably really didn't have a need to boot to OS X since I was working from OS environments loaded from the optical disk.
With that being said ... I am currently running non-Fusion with separate SSD and Hard Disk and managing the files manually, although I am tempted to rejoin them as a Fusion sometime in the future.
Anyway ... my $ .02 🙂
-howard