Neck angle adjustment is used to change the height of the strings above the fingerboard (called action) to accommodate your playing style.
When I was tinkering with prototypes, I stumbled upon the crocodile-jaw-like mechanism you get when the neck is hinged to the body. Like the cartoon-idea of the open jaws of the crocodile, I placed a ‘halter with bit’ (a steel tube) in the beak of the neck joint, to prevent it from closing. The steel tube is pulled into the back of the mouth by a screwing bolt which can be adjusted with a hex key on the backside of the eub. This mechanism changes the neck angle and so the stringheight is adjusted.
Steel tube prevents ‘jaw’ from closing
Hex key adjustment on the back
Why not at the bridge?
In general, when working with a bridge and a tailpiece, there are two methods to adjust the action.
One is to make the bridge itself adjustable; permanent by shaving off wood, or reversible with thumbwheels in the bridge for instance.
The second method is to slightly alter the angle between body and neck. The angle change to get the strings 1cm closer (which is a lot!) to the fingerboard is very small (about 0.7 degrees), while one cm extra bridge height is a big alteration to the bridge. Hearing a difference in tonal character after a neck angle adjustment of a few millimeters is unlikely.
I’m not too fond of alterations to the bridge, since the bridge is like a bottleneck where all energy that is exchanged between strings and resonator block passes. Also the piezofilm pickups are directly under the bridge… Changing the stability and stiffness of the bridge may alter the character of the instrument.
I’m not saying one method of adjustment is better than the other. In this particular design altering the neck-angle just seems the better option to me, because I think with this method a change in the character of the tone is the least likely occur.