On the other hand... I like the flavor of allowing just a reasonable amount of that. Archivists are supposed to be constantly searching for obscure scrolls; having them looking for spell of obscure dead diety X or Y is very much part of the class. You just don't want to let it get out of hand... obviously, Archivists shouldn't be able to just walk into a scroll store and buy every scroll on every domain list everywhere, plus every spell on every non-cleric divine caster class.

The thing about learning obscure spells is that they're obscure. Getting them too easily would ruin both the balance and the flavor of the class.