Hi Aaron,
I went back and tested this situation below, and currently the following definitely happens with large maps--design map size of 64x64. It does not seem to happen with smaller sizes--I also tested this with a 16x16 map and it worked just fine--paint on the terrain, see the painted area in real-time.
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So, on a 64x64 the attribute brush does not work correctly:
The attribute brush is not updating the map until I close the attribute brush window and then re-open the attribute brush window. In other words, it shows nothing while I paint with it--say adding snow here and there, then I close the window, and it, of course reverts to showing the usual 3D image, then if I re-open the attribute brush window, the changes show up when it switches to showing the attribute map.
So currently, there is no way to see where I've actually painted in real-time.