As a data point here ... Kelly plastic mouthpieces for trombones and euphoniums can be had for $25 with free shipping (e.g., from Musician's Friend), or roughly 18 euros. They have the advantages of being professionally finished, available in a variety of colors, and quite faithful copies of the originals. So it probably wouldn't be worth it to copy something that Kelly already has .
However, Kelly has copied only the most commonly used mouthpieces of what might be generally referred to as "beginner" or "intermmediate" in nature; and so the selection is limited. I certainly have a couple of tuba mouthpieces that I wouldn't mind having plastic copies of.
I'm unclear about what the legal issues may be in the copying of such products. If you do some research, you'll discover that at least SOME mouthpieces currently being sold are patented. So copying one of these would be a patent infringement. It would be interesting to know whether Kelly (and Faxx and others) just outrightly copy their product line or license the right to do that. The overwhelming metaphor used in 3D printing is that of (duh) printing, and so sticking with metaphor, one would expect the intellectual property rights to fall along the lines of copyright rather than patent. But I don't know if you can copyright something like a mouthpiece. Certainly you can copyright the document that describes the DESIGN of the mouthpiece. But then producing a physical object (by 3D printing or otherwise) conforming to that design seems more akin to producing a meal according to a recipe than it does to copyright infringement. I know someone who runs a company that develops and sells 3D printers (and supplies materials). Will shoot him an email and see what he knows about this.
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)