In the same way we can override the PlayerController class, GameInstance class, etc. is there a way to set a default blueprint class to use instead of StaticMeshActor?
No. I’m not sure that even makes sense though. You can create your own class of static mesh actor if you want, just like any other class. Then you could use that where you wanted. But there is no such thing as a default static mesh actor. You would simply create a custom one and use it anytime you wanted.
The idea is to have custom functionality for each mesh imported into the project right after importing, instead of wrap each mesh with an appropiate Blueprint class after the import is done. Does it make more sense?