[Note, this is very rough, just to capture my idea for later perusal. Expect grammatical/spelling errors, non-sequitors, etc.]

Alexander Green asks:

"Okay hive mind-- how would you approach this project?
We are well underway but I thought some of the folks on this group may have good ideas that we haven't thought of.
We need to prototype and manufacture 15,000 realistic leaves with 5-10 addressable LED's per leaf (Apa102)
I'm not interested in your prototyping technique if it doesn't elucidate a plausible and economical manufacturing pathway. (Injection molding some sort of plastic is an obvious solution but it's hard to predict cost and likely will be very expensive.)"

https://www.facebook.com/Green3r/posts/10100799256235928

Here's my response:

The basic idea is that you "sew" a pattern of LED's into a flexible fabric, which you then glue to the 3-D armature of trunk/branch/twig.

• Design the armature (trunk, branches, twigs) for the tree. Use whatever software hydroprinter's use to predict how to transfer 2-D image to 3-D object to predict how to "glue" leaves to branch/twig armature:

https://www.youtube.com/watch?v=MeWvYP5AHqI

• Lay out your circuit pattern using circuit diagraming software.

• Buy the material they use to make outdoor silk flowers. You'll need two kinds: a sturdy green UV resistant top-side fabric (to form the visible leaf), and a thin, translucent "veil" to form the underside.

Sunbrella is a UV-resistant, color fast fabric used in the marine industry. It's generally pretty thick fabric, but maybe they make a version that's fine enough for your purpose:
https://www.sunbrella.com/en-us/fabrics/collections

Or find out what these guys use:

Polyblend
http://www.autographfoliages.com/ProductInfo/PolyblendOutdoor.aspx

Veil:
http://www.aurorasilk.com/silk_fabrics/illusion.html

• Stretch green fabric over a canvas stretcher, as if you're preparing it for painting. Like this:

https://www.breathingcolor.com/blog/4-options-for-stretching-canvas-prints/

• Soak the canvas in a starch/PVA bath, then let it drip dry. Ideally, this will make it rigid. Like this:

https://www.engadget.com/2016/09/09/robot-sews-a-shirt-automatically-using-stiffened-fabric/

• Once dry, remove the now stiff fabric from the stretcher.

• Sew your circuit diagram into the rigid fabric with conductive thread.

• Use a pick and place machine to insert diodes into your sewn circuit, like this:

https://www.youtube.com/watch?v=WfK185zO7o0

• Glue the translucent veil over the top of the completed circuit.

• Use a laser cutter to cut out your leaves. Like this:

https://www.youtube.com/watch?v=HGRVoR0Y5bA

• Glue the cut pattern onto your armature.
  For the armature, I'm envisioning a giant version of a children's push puppet toy:

https://www.youtube.com/watch?v=OVAM4E8OLio

The idea is that you assemble all of the pieces of the tree on the ground, threading through a post-tensioning cable through all the segments. Then, when you're ready, you tension the cables to cause the tree to assemble into it's final, rigid form.

Like this stair:

http://www.archdaily.com/799954/see-the-engineering-behind-this-floating-award-winning-stone-helical-stair

• Find a real tree or trees whose form you like.

• Use a 3-D scanner to scan attractive twigs and branches.

• Make a 3-D model of the final tree (Z-brush maybe?) using your scans. The goal is to create a 3-D model that you can use either to make a male plug or a female mold for later casting:

https://www.youtube.com/watch?v=qDRN4VZ8oTo

• Section the model into transportable pieces. Also insert raceways for wire and the tensioning cables, and insert hard points for craning and forklifting the segments. Maybe divide up the tree into cubes, with section designed to fit into a cube scaffold. You can use the scaffold to transport and protect the segments. And maybe it would be useful for final assembly as well--just stack the cubes in the right order, supporting the tree, until the tree is ready for final assembly.

• CNC mill / 3-D print male plugs for each of the sections out of foam. Maybe use Foamlinx in Sunnyvale:

https://www.wecutfoam.com/single-post/2016/12/06/How-We-Fabricated-Evil-Knievel-Foam-Sculpture-Prop

• Laminate the male plugs with basalt mesh and CSA cement (you can also use fiberglass, but basalt mesh + cement is nicer to work with than fiberglass), and it's not as shiny. Keep applying layers until you reach the strength you need. At this point, you'll probably also want to attach armature wire for the thinnest twigs.

• Paint base layer. (Skilled airbrush artist is probably the best method.

• Glue on your LED fabric circuits (see above).

• Paint final layer, as needed.

If you want a very fine finish surface, you an CNC female molds, and laminate the female mold, and then fill the void with foam.