Watching a tutorial online for Substance Painter's new tools, I was intrigued by the way they can use a secondary widget to define the orientation of an object's symmetry such that they can move the widget and then paint a symmetrical shape on the object's surface. This is similar to how our paint system uses the optional X, Y and Z axis for symmetry painting. Is there a possible work around in Blender already that I have missed?
I saw a Kent Trammel video on youtube that partially gave clues to how this might be possible, so I set up a basic scene in 2.8 to try my hand at applying the sculpting workaround to Texture Paint. You can see the normal symmetry painting there in the first screen.
I then scale the Plane object down once we have it in place - the local coordinates of this object are what we will use to get our symmetry set up. I modified the plane in edit mode to extrude after creating the two loopcuts - these will be a cross that will help see the center of symmetry.
Select your paint object and return to Texture Paint mode (Face Select Masking on to avoid painting on the opposite side of the object) and paint your desired design - here i test with just a cartoon face, as if Suzanne has a poorly made tattoo on her face like a rap artist. Painting one side of the cross hairs gets me the symmetrical result on the other side based on the helper object's local orientation.