"I thought you guys might be interested in a little weekend project of mine. I fabricated a clip that attaches the Wiimote to the Classic Controller, using the 'mysterious' 3rd shoulder button.
"I've always hypothesized that the 'mystery' button (un)connects an as-of-yet unreleased clip that would allow you to attach your Wiimote to the Classic Controller. The idea being that such a thing would be good for (a) getting the Wiimote out of the way and (b) for any games that may want to have a mix between dual-analog controls and motion-sensitive controls.
"I'm working on my PhD in mechanical engineering, and as such I spent some time in my research lab over the weekend working on graduating. I figured that while I was there, I'd spend some time trying to bring this hypothesis to fruition. So, using the lab's Stereolithography machine (a type of rapid prototyping machine), I made a clip. It took about an hour to draw in CAD, and it took the machine about 2 hours to make it.
"You feed it a CAD model, the computer slices it into cross-sections, and then the part is built one cross-section at a time. SLA does this by tracing a UV laser over a vat of photo-polymer resin -- a viscous material that solidifies once it is exposed to concentrated UV light. One layer is made via a series of laser scans, the part is lowered deeper into the vat, a new layer of resin is spread over it, and the process repeats itself. "