I ended up getting this working actually.
My steps:
- import a bunch of textures into unity (head, torso, forearm, upper arm, etc)
- make a SpriteAtlas that contains those textures.
- make an asset bundle that contains the SpriteAtlas
- at runtime, download the asset bundle, load the SpriteAtlas, and get the Sprites from the atlas.
- pass the sprites to Spine to packed up
The thing that was tripping me up is that you have to go into the Editor Settings and change the Sprite Packer Mode to "Always Enabled". If you build asset bundles in a separate Unity project, you must set it to Always Enabled in BOTH projects for this to work.