TheZero I know your question is about the BoneFollower component, but from what you want to do, you seem to be taking a rather roundabout approach to begin with. Spine skeletons can be tinted or have different materials applied to each slot in Unity. If you have not read the following section, you may want to read it first and consider how to modify your skeleton directly: spine-unity Runtime Documentation - Changing Materials Per Instance
Even if you still want to use sprites that are not included in the texture atlas, it would be better to replace the slot attachments as shown in Spine Examples/Other Examples/Mix and Match Equip.
This way you can reduce draw calls by collecting the images used in your skeleton and repackaging them into a single texture.
I think it makes sense to use the BoneFollower component if you want a particle to follow a specific bone, or if you want an image that can be attached or detached (like picking up or dropping a weapon), but if you want to replace limbs or other parts that basically never leave the skeleton, it is better to replace the attachment in the slot or override the material per slot, as this should be less likely to cause drawing order problems and more manageable.
