Just found something and I hope Teyon sees this!

The Mirroring problem with the Morph Brush appears to be down to the figure.  I noticed today that the figure isn't welded properly at the seams even inside Poser, and it's those points that aren't welded that are causing the Mirror to fail.  If I use the Advanced Body Control to lift-up Roxies arms, use the Smoothing Brush around the arm pit area, Mirror it, and then hover my mouse over the Mirrored side, I can see double-vertices where the mesh isn't welded (see attached image).

If you look closely at the highlighted vertices, you'll see two rows of them in some parts, this is where the Smoothing Brush has smoothed them apart before doing the Mirror, and they only parted because they aren't welded.

The Poser figures seem to suffer this random unweldedness on just about every seam, not just the arms, and not just on Roxie, it's all of them so I can only imagine the software being used to create the welds isn't working properly.

This splitting thing has been a pain in the backside for some years now.  And it's not just splitting, there's an edge issue around the seams as well.  Best way to test it is to export the figure from Poser without welding the seams, and then import the obj into ZBrush and run the smoothing brush over it.

Ugly splitting and hard edges will make an appearence on Poser figure seams, but not so with DAZ figures.  Whatever is wrong in the creation process needs rectifying because it's a pain in the backside from a content development point of view, I really hope Teyon will fix whatever it is that's causing it.