@KevinOrtman, thanks to you and the others this newbie is having some success. I have plinked around and created a few test interfaces and successfully gotten overlay buttons, sliders, and other oddball graphics to overlay, (simplistically, I know). I learned enough git to get the beta branch down and working, too. The Pi + Reality Editor solution is really surprisingly robust in finding targets and applying overlays!
My next goal is to control a white Hue (dimmer) with a slider. I've got the control slider, but am unsure where to start to tie in the Hue REST interface (curl scripting?). Does it all go into index.html -- that seems wrong, the object control should talk to the Pi, and the Pi should talk to the Hue? I looked through the Arduino Yun Example for twiddling analog outputs for a LED, and I gather the Web part at the bottom goes in index.html to add some meat to the slider object overlay. But where in the Pi code does the back-end go? There is some Arduino code in the example, but no file identifier, and the helpful link to instructions goes to 404.
If you have time to advise, I would much appreciate it, else after some errands today I'll go read some of the Pi code framework and see what I discover.