The only legal issues I can see would be getting bureaucratic approval for the operation of the lens. The DOT requires specific performance values for light pattern, light intensity, color, and reflex. Getting approval can be a pain in the ass since you would be dealing with bureaucrats who's function in life is to be assholes.
As far as meeting specs, it would be easier if they were molded with the original tooling. Material isn't hard as most material suppliers have resins that meet specs for color and optical properties. The only problem might be in setting up the quality auditing stuff to test the lenses as they are produced. Dark tunnels, calibrated light sources and sensors would be needed to test performance. Again, these might be available with the tooling as scrap.
What does the BMW supply chain look like? Are the lenses produced by an outside vendor? Does BMW even own the tooling? If BMW has no interest in purchasing additional production, would they be willing to release the vendor to produce parts for the aftermarket? I know that domestic auto makers outsource parts that require specific expertise rather than try to compete with companies that can do a better job at lower cost. Look at all the Bosch stuff that BMW uses.
I am not sure that 3D printing is a viable process to make lenses. A lot of the optical properties of a lens are dependent on the density and internal stresses of the plastic material in the final molded part. 3D printing probably results in minimum molded in stress, but how can density be controlled? Also, are there any 3D resins that have the required color and clarity? I suppose that there are acrylic resins adapted to the 3D process, but have they been tinted to the approved color?
Last spring I had contacted a laser measuring company for a quote to develop a data file for printing RS lenses. The price was estimated to be in the $8-10k range per lens. Amortization over a 5 year period would probably add about $20 to each lens. Add in costs for material, production and distribution, and you have an expensive piece of plastic.