I found a way. Sorta. I added a new menu option to Assets that allows me to save a mesh to a file. So, the workflow is:
* Create the sprite batcher
* Add sprites to it, position 'em
* Click commit in the sprite batcher
* Add a mesh collider to the sprite batcher
* Save the mesh (Select the sprite batcher, then go to Assets -> Save -> Mesh)
* Delete the mesh collider from the sprite batcher
* Add a new empty game object, make it the parent of the sprite batcher. Fix the positions.
* Add a mesh collider to the new parent. Drag the saved mesh to the mesh collider's "mesh" selector.
You'll need to repeat the last few steps every time you change the sprite batcher contents.
I've attached the script I made. It's my first editor script, it may not be right, but it's a start.