Is there any way to only symmetrize the masked part of and object in sculpting mode? The Remesh->Symmetrize in dyntopo option will symmetrize the whole object even when some part of it is masked or hidden.
Long time since it was asked but for others looking for an answer. If you go into edit mode and recreate the mask with by selecting the relevant vertices, then go to Mesh-> Symmetrize.
Strong recommendation that you are in wire-frame and locked looking along your symmetry axis when making your selection or you can get some weird results.
Either box selection or the brush selection (C key) are pretty useful. You may end up with some holes in the mesh where your selection ends. Just select the edges of the hole, fill and then run over it with a dynatopo draw brush on 0 strength to hit the necessary geometry density.