I think I answered all my questions 
This isn’t the most efficient approach, but here’s how I got it to work.
My model (the bush) was skinned to one bone and exported.
The particle emitter had the following settings:
- Under required, I selected a material for the leaf particle. I set screen alignment to Facing Camera position and I set max draw count to 100. I need to check this as the count in Cascade still goes up to 1000.
- Under Spawn, I set the rate distribution to constant of 0. I added an item to the burst list where I set count and count low to 100 each.
- I added a Skel Vert/Surf Location property. Source type: Surfaces.Orient mesh emitters. Actor Param Name “VertSurfaceActor” (unchanged from default). I assigned the skeletal mesh here from the editor. Under associated bones, I added one entry for me single joint in my model (in my case it was named joint1)
- Lifetime: Min and Max = 0. I think this should have infinite life and with the other settings should not increase over time.
- Initial size : up to your discretion: I set it to 10 10 10 for the leaf particles.
- Initial velocity : Set these to 0 to prevent the particles from flying off.
To get all of this to work there was one important step I was missing.
Place the mesh and the particle emitter both in the scene. They can both be at the root level - parenting is not necessary.
On your particle emitter, go to the details tab, go to Instance Parameters - add one element. Name = “VertSurfaceActor”. Param type = “Actor” and assign your bush mesh to the “Actor” field.
That should work. If it doesn’t you might need to check things like initial size etc. Hope this helps someone else!