Quote - You can mirror morphs on it using the morphing brush.

Great news. Remember I'm still soldiering on with Poser 7 so I'm never quite sure what the later versions can or can't do. ;) 

Quote - I was working with obj morphs and not those made by the morph brush.

That wouldn't matter if you exported the 'brushed' morphs as OBJ, although a bit fiddly I grant you. Not a great workflow if you have a bucketload of morphs to do.

I did have some success in the distant past using The Tailor (remember that? :) - create a morph on one side, mirror the mesh then transfer it to the other side. It occurs to me that Morphing Clothes ought to make a better job of it, but it would still be a fiddly process.