Ok, I have an update on the creature shearing behavior that I thought I might share.
It happens that the issue I was having with the sheared tissues (ivory, from bodypart:tusks) was based on both the creature size and the relative tissue thickness. When I increased the creature bodysize significantly, the sheared ivory tissue became usable for crafts and similar reactions.
Also, with the increase to the creature size the sheared ivory tissue showed up inside the workshops not as "Creature" Ivory, but instead as "Creature" right tusk and "Creature" left tusk. It appears that there was a critical size where the removed tissue satisfied a requirement to count as a complete bodypart, and thus could be used in reactions. I am not certain where this critical size is, and I am pretty sure it is dependent on the body plan, tissue shape (layer, strands, feathers, scales, ect.), and tissue thickness (BP_RELSIZE and TL_RELATIVE_THICKNESS for example... I think).
Speaking of relative thickness, by not changing the creature size and instead cranking up the tissue relative thickness (by a very significant amount, like 100x) the same result was achieved; the sheared ivory became usable. I don't know absolutely, but I think tweaking the body plan to change the RELSIZE of the tusks to make them bigger would allow a less outrageous TL_RELATIVE_THICKNESS value. !!SCIENCE!! is still needed.
All in all it appears that, very much like butchering, when shearing a tissue there is a minimum tissue size that must be met to satisfy requirements to count as a usable bodypart. Right now it seems that any means to define or modify the tissue in question to make it large enough to meet the currently unknown criteria will work, the most simple being to increase the body size of the creature.
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I did some quick tests on normal creatures with shearable hair to confirm some of these observations, and sure enough, decreasing the creature's size to very small made the sheared hair unusable to make yarn.