reversing seat post

It's more than likely safe (you are changing the stress on the seat post, but FSA doesn't really make anything super light so you should have enough material to withstand the change in force).

That being said, why are you moving your position? You're much better off changing the position of your aero bars to you, then you to the aero bars…. Think about it this way, the first parameter you fix in a bike fit is the riders position in relation to the bottom bracket and then everything else is adjusted from this fixed point.