How to patch a wood lath plaster rounded corner?
Since this got ZERO love, and I have finished the job, figured I would post what I did.  

Simply bought some wire mesh from the local home goods store, and screwed it directly into the wood lathe after breaking out the hole a bit more.  That gave me a good base for my 20 minute hot mud to mushroom through.  As for the rounded corner -- it was nothing special, just took a minute or two of using gentle rounding strokes to make it match the existing radius.  

In hindsight, probably give it a minute to set a bit more before doing the radius.  Also -- I needed to apply the mud even CLOSER to the outline of the electrical box.  I left it a little shy of the box, and the regular sized plate cover did not cover the hole, and I needed 1 more trip to the store to get a 'JUMBO' cover plate.  Turned out great.