Put another way, travelling FTL in and of itself does not mean you're travelling backwards in time. But the geometry of the universe provides a barrier between STL and FTL, yet no barrier between forwards and backwards in time except inasmuch as you need to first rotate from STL to FTL before you can rotate from forwards to backwards-

So if you invent such a barrier, then sure, FTL is fine. It's just that there's geometrically nowhere to attach that barrier, except if you make use of the CMB.

The CMB is not a reference frame - everyone sees their own CMB.

That's a fascinating thread. Thanks for the link.