I have it working, but man it was a process. I wish I could tell you exactly what I did to get it working, but I didn't write it down. :( I do know my first 10.10 attempt was glitchy. Since you can't do full upgrades (like 10.10 -> 10.11) using these hacked-up images, I started over with a 10.11 hacked image, which worked a lot better.
It's a combo of using a hacked image from gray areas of the interwebs, and then messing with image settings outside of the image itself (using VB commandline tools). I'd do a search for instructions anyone has posted since 10.11 since I would've used one of those.
Disclaimer: I don't do this at all related to my FreeNAS box. This is VB running on my Linux workstation.