Cheers for the responses. I wouldn't even call that a proxy Onyx - it's just a scratch built Lord of Battle
Going by tomkatt's list, which seems pretty complete to me...
On points 2 & 3 I think it's okay. I won't be making it something different each week, it's going to be a LoB and only ever a LoB. And while the Feral Titan is a bigger model than the LoB, the LoB has double the DC of the the titan, so in terms of scale it's a better match than the original model.
But it's point 1 that I'm worried about. The only 3 non-daemon war engines in the army are the LoB, the Chaos Altar, and the Plague Tower - so it's obviously not any of those things. But there's a lot of Chaos lists, and plenty of them have Feral Titans in them. I guess I'm concerned about someone thinking it's a Feral Titan part way through a turn, especially because Feral titans are very different units.