Given the recent revelation that this item is in game and currently discoverable, I would like to open up a discussion to help solve this mystery.
It seems that simple crafting research alone is not responsible, given that a few players have confirmed all crafting research done and no discovery. Therefore, we must assume that the secret lies down some other path. The first clue would be in the actual description of the bow:
"The manufacture of the Bow is not
merely
a mechanical task, but is in many ways a spiritual process. And likewise, it's use is meditative as much as martial."
From this information, one would assume this reinforces the fact that research alone will not unlock the secret. Furthermore, there are additional descriptors that I feel reinforce this as well:
"Wise and reflective elf" and it's arrows "Ride the sunlight"
Now looking at known map locations I feel is a start. And the first one that seems to fit the bill would be the Temple of Reason 295/-165.
I would ask that anyone else express their thoughts or ideas, as I am not by far the most knowledgeable person on special locations in Illy.