Well in addition to it being quest locked, Eyevea is essentially the property of the Mages' Guild and would it make sense for them to welcome mages of all sorts of other schools (and none) into their domain?
Eyevea is a great idea, I hope you follow through and do this
You could always say they're helping host it or something, and quest locked is better than DLC locked. The only other thing I can think of would be Craglorn or something for neutrality.
Well on the one hand I LIKE quest locked areas just because they tend to have less random players in them in general and if you give everyone sufficient lead time they can totally get the quest done beforehand on the character they want to attend with. And you COULD say, for purposes of the event, that it isn't Eyevea at all but some other privately mage owned island.
The other location being considered is the Observatory in Belkarth in Craglorn so yes it has the advantage of being easily accessible and ICly neutral. Honestly, thinking about it, either could work well.