I am not familiar with 3D printing so allow me a few questions.
First, can the process make transparent parts? I was under the impression that the resins used were opaque.
Second, is a UV resistant dye available to color the resin? I suppose an amber bulb can be used with a clear lens as an alternative.
Third, can the printer work in acrylic. This material has the optical properties the OEM lens was designed for. Light dispersion properties will be degraded if a different resin is used. In fact, even if acrylic is used, the molding process also has an impact on optical properties.
I have wondered how many lenses are needed to supply demand. If indeed it is possible to create a digital file of the lens(needed to make a 3D copy) it would be possible to make an injection mold and produce a "prototype" run of a couple thousand parts for a price lower than what BMW charges.
These parts would be identical to the OEM parts, depending on how accurate the digital "scan" of the OEM part was. The problem I see is that who ever produced them would be out of pocket $5-10K and would have to sell a lot of lenses to recoup the original investment.
It would be great if someone in Europe could get their hands on the original tooling and do a production run. They are close to the original, and some detective work should be able to find the tool that made these parts. I know it can be done because I have dealt with businesses that make parts for the auto restoration market that do just that. Someplace in the BMW organization is a person who disposes of old tooling. If I knew who that person was, I could start a very nice little business.