I am running a fusion drive setup and at first, this fix did not work for me. I was able to get it working by using Chameleon Wizard to do two things: first, I generated a new serial number and profile in smbios, and second, set my default drive (in org.chameleon.boot.plist) to my SSD in my fusion drive, which is disk (0,2) in Chameleon/Chimera. I then copied my /Extra folder to the "Boot OS X" helper partition on my SSD (disk0s3 on my machine). Just to make sure, I also extracted and copied boot, boot0 and boot1h to the correct places using the commands in Neilhart's guide. After a reboot, I was able to log into Messages and it is working fine. I know a few of you have fusion drive setups and I hope this is helpful to you. Like many of us, I've been working on this for a few evenings trying to get Messages working!
My build: GA-Z68XP-UD3, i7 2600k, Radeon 6870, 16GB RAM, MacMini 5,1 profile