I don't think they'd have a problem with an Assist Trophy item for Fire Emblem characters, if it means to them that those Fire Emblem characters won't be getting in as playable.
The AT argument and pettiness aside, one of the huge positive parts of the Breidablik summoning the characters is that it would mean alot of the stuff that makes up Fire Emblem that's not in Smash get represented, like Mages, Armored Knights, Pegasus Knights, Cavalry, Wyvern Knights, Manaketes, Beast units like Taguels or Laguz, heck, the Breidablik can even summon Gods, Goddesses and other beings on that level. The godly beings would practically be the equivalent of getting a Legendary Pokemon from a regular Pokeball or Rathalos from an Assist Trophy.