i discovered this and actually figured out how to do it over a year ago. first what you have to do is summon the creature then go through a door that creates a loading screen (i.e. loading enter a cave) but make sure that the creature didnt follow you by summoning it away from the door then running to the door and entering then if its not inside when the loading finishes just reload the autosave from entering the door then go back outside and if done correctly you will have a nonhostile summoned creature.
notes:
this doesnt work on all creatures (i think it works best with shambles and hungers from the shivering isle dlc/plug-in)
the permanently summoned monster will be hostile to guards or other npc's
the permanently summoned monster will disapear if you do not visit the area within three days (similar to items resetting that are stored in boxes) this is because the area resets every three days if you dont go to it
obviously its been a while since the last post but not terribly long so hopefully you will see this and if you have any questions make a post. But i dont guarantee i will see it.
Btw all credit goes to me i looked when i discovered it this glitch for like an hour and nobody had made a post or attempted to let others know, so im the first
