Given Yoshida's your uncle and you gatekeep the suggestions on these boards for him, one would think that you'd know this wouldn't be a job for the animators
It was confirmed in the past that FFXIV uses Havok for physics and most likely animations as well. Havok has had built in animation mirroring since version 5 some 15 years ago. However it's not something I've ever seen the game use, kind of surprising given that it's a good low effort way to somewhat obfuscate frequently recycled monster animations and postures. So there's going to be some underlying reason that it's not been done already at this point (assuming I've not simply missed a monster that uses it of course!), probably spaghetti code related but who knows for sure.
But yeah, it's 100% a code issue, not an animation thing.
Personally, I think it'd be neat thing to implement if it could be done without breaking things. It'd be a nice boost to FFXIV's character customisation.