I think a guide to setting up a Fusion Drive including the internal SSD and an external HDD would be a great idea to have on IFIXIT. It’s not really a hardware thing, but it sort of is, and it would be in the spirit of iFixit. I don’t have one of these (yet) so I won’t be writing one up too soon. Feedback on the idea please! Perhaps this (VROC-Virtual RAID On Chip) is what’s behind the non-upgradable SSD points @danj makes?
I think the YouTube video that shows how to do it with dry ice is better than this video.