Hello,
I totally know what you mean! It happens a lot.
In that case, I usually duplicate the original mesh and move it to a new slot, and delete some vertices on the mesh’s hull, then put it above or below the slot that I want to sandwich.
Mesh attachments can be shaved the outline by modifying the hull, so it allows separating an image in the Spine editor.
The good point of this method is that it allows you to use a mesh that has exactly the same weight. Clipping masks can use weight, but it is tedious to match the vertices and weight with the original mesh.
Also, even if you duplicate a mesh that uses the same image and place multiple meshes in the skeleton, that image will not be duplicated in the output atlas image, so it will not needlessly increase the size of the atlas.
In your case, it would be better to duplicate the mesh of that backpack and put one above the legs and put one under the buttocks, or duplicate the buttocks mesh and cut out the buttocks mesh just for the part of the backpack that needs to be hidden, and put it above the backpack.
If it penetrated between other mesh when you animated them, create a new bone for adjusting it and modify the outline.
I hope this helps you.
