Please remember that these are rumors. Also please remember that there are multiple sources who are still saying that a 40k Skitarii codex is coming, and I believe from what I have been told that its not only credible, but mostly complete at this time (although I had thought its release was still some time off). But who knows, and I want to make sure that if a source is saying that he is not aware of a Mechanicus codex, that the readers here on the site get this information.
I do find great interest in the Horus Heresy doing a Dark Mech army list, and feel that its just a matter of time before we see this.
Unfortunately there will be no 40k Mechanicus at this time, the only
upcoming projects I am aware of is for HH.. But there will be some twisted
Dark Mech models coming out, but again for HH.