I use the Build plugin to facilitate buildiing large and complex structures. I realized you can't change ownership fully or partially on a structure wiith owner Unknown at any %. Build doesn't seem to associate a player with the spawned entities and because of that I can't claim ownership.

Hm,
you can use "/own" on foreign/unowned structures. (works for me)
you can use "/setowner Kaizer" on foreign/unowned structures. (works for me)
you can use "/unown" to remove any owners. it will turn back to "Owner: Unknow: 0"

But only for the entity you are looking at. The "/own all" doesn't work for me either. Plugin has troubles finding surrounding entities.