I don’t mean to hijack this thread and refocus it on 3D printer technology - but I do have a couple of thoughts to share about a printer with 6-axis capability.
Changing the nozzle diameter would be a very tricky mechanical problem to implement. Plus doing that would greatly affect and complicate the calculation of how much filament to extrude at any given point. Because of this my sense is that keeping the nozzle diameter fixed and varying the extrusion amount would be a much better and simpler approach.
Varying the amount of filament that is extruded is done today by a couple of slicer programs, but they do it via the trick of variable layer height. Here’s my blog page about that if you are not aware of the method: Variable Layer Height. The technique does work well for parts having the proper type of design, but in general the variable layer height technique could never compete with a 6-axis printer with appropriate code controlling the amount of filament extrusion.
If you were going to use a 5-axis milling machine with a ballnose cutter to mill the constant diameter elbow pipe shown in your first post out of solid material your toolpath generator would have to space cuts much closer together on the outside/large curve than on the inside/small curve. This is independent of how the actual geometry of the part is defined, and that suggests to me that the 6-axis printer should have either the ability to change layer “height” as it makes loops around the curve, or that it needs to be able to change the amount of filament it extrudes as it prints. Or maybe both.