I'd be great to select edge ( on simple low-poly object ) or draw a straight line curve ( hi-poly model ) to enforce custom axis on local symmetry.
ex. lets say You have some sub-meshes in your character, not symmetrical positioned with little offset from global x=0, and they are roughly, but not full topo-symmetrical. Actually the only way is export separately submesh and reorient it's pivot in external software ( ex. maya ) which is long process.
Going further if You want to reuse You could preset / manage your custom symmetries.
