Thanks for the link. Might still be easier to call it a separate L2, because a Serbo-Croatian-esque solution will look stupid here, giving way too much precedence to a basically extinct sublect.
That's why there were separate categories for it, but you deleted them. I would compare it more to Old Church Slavonic in Glagolitic. It was never as common as Cyrillic (except in Croatia) and most textbooks on OCS consider only the Cyrillic variety. But we still consider them one language.