The reason I kind of like the gear on the tailpiece, as much as it's an in-ensemble tuning inconvenience, is that it doesn't cram the pegbox the way an additional tuner in there would. Obviously, many 3- to 4-string conversions make it work, but it makes string changes and forelength logistics a real challenge.
Martyn Bailey is an expert of the highest order. The Thomas George Martin 5 string basses are gorgeous, I would love to have one. Here’s an old fiver from Mr Swan’s shop. However …to me, the modern solution for the classical bassist is just tune in 5ths. C, G, D, A. Don’t need five strings or extensions. If you want a high C like the old Kay 5 strings, for whatever your reason, go E, A, E, C.
I am currently engaged in constructing a couple fretted bass guitars at the moment, rush job... however, after considering all the interesting ideas, I would have to say I am leaning toward the tailpiece solution, as in the photograph in post #22. I do wonder what the hole in the tailpiece below the tuner is for... hmm. Perhaps a previous attempt at the same idea?