TexTools uv symmetry preview
plain topology based UV symmetry script that mirrors one side to the other. At the moment it works best with quad objects and of course symmetrical 3d polys or meshes.
It works like this: either select 1 edge and it will edge-loop the selection and set that as the mirror axis. Or you select 1 half (either edges, faces or verts) and it will calculate the mirror edge axis for you and mirror it to the other side if possible.