[GraemeJ]

Well, the 12.5" is a dealbreaker. It's the concept I'm looking for, but not big enough.
Also, I do not want to lug around the bottom half of my existing Manhasset stand. The bottom half of the Model 52 would be a lot more mobile, but I do not see where you can just buy the lower half.

I imagine their 12.5" refers to screen size, not the case, so it might well be that a 13" tablet in a smallish case would fit.  I'd contact Manhasset direct and ask them what are the actual maximum dimensions the holder will accept?

To me, the bottom half of a Model 52 stand is basically like many mic stands - so buy a mic stand, plus a suitable tablet holder.