I have been trying to think of a way that one could make a built-in capo device that would allow you to capo the 8th string at the nut, effectively making the nut extend over 8 strings instead of just 7.
One idea might be a wing screw device. What I'm wondering is whether that would necessitate inserting a sleeve into the soundboard at the nut level where the wing screw would be inserted. As you turn the screw it pushes the string down.
The ideal solution would be to have a luthier remove the extension for the 8th string and making a new nut that goes across 8 strings. But that is too much surgery. Thus the concept of just capoing that one string, hopefully with some sort of built-in capo device.
Needless to say, the reason for doing this is because there are many times where it would be convenient or necessary to fret the 8th string.
Edit: Here's an interesting idea. See the picture at the bottom of the page.