In the meantime, until I get the motivation for taking the T-beam guitar some steps further, I might as well plan even more. ...and consider alternative methods for future guitars.
One of the issues I prefer to think of is the possible tuner systems for headless guitars. On Building the Ergonomic Guitar, there's much discussion around how to make a headless tuner system. See e.g. here and here.
I'd like to add another possible principle to the discussion: Using a bicycle brake cable tensioner. It is normally used to tighten the brakes of a bicycle by turning the nut with the rubber ring seen here
(click the images for a better view). Since the threaded, cylindrical part of the tensioner has a flat area which fits a hole of the same shape in the brake arm, it will not turn (but only slide) when the nut is rotated. On a bicycle, as seen here 
, the outer cable stops at the tensioner, and the inner cable goes through it and is fastened on the other arm of the brake.When using it as a guitar tuner, the string passes through the cylinder (the part with the outer thread)
and the total of six tuners/tensioners should be mounted in a brace with six holes of the shape that allows the cylinders to slide, but not to rotate 
. By turning the individual nuts (the part with the rubber ring), the strings are tuned.Of course, there's no need to use dedicated bicycle brake cable tensioners. It might be better to make the tuners from pieces of threaded rod with a hole drilled through them plus some knurled knobs such as these
. Choosing three knobs of each of the two types shown in the picture would even allow for a staggered placement of the knobs, in turn allowing bigger knobs than with an in-line configuration.I'd expect it to look like this.
It seems to me to be one of the simplest solutions to the problem of making cheap, accurate and reliable tuners for a headless guitar. I may have overlooked some problems with the principle, so prototyping must be the next step. Once I've finished the T-beam guitar.
