Defaults for variables of asset/object/class type do not show up in the defaults panel in User Defined Structs. This severely limits their usefulness as a data definition.
Create a user defined structure
Add a new variable
Change the variables type to Static Mesh
Observe it doesnt appear in the details panel to the right.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-11714 in the post.