Ah yes, thanks for the link! No wonder I didn't know about it, didn't join the forums before this spring, after all.
I think your idea is lovely, especially considering the context in which tapas has to operate now. A rating system like that could easily be expanded, as well: not only for age groups, but thematically. So you could have, I don't know, "Young Adult + mild nudity" (let's be real, YA protags need their sexual awakening), making sure it is NOT flagged as "NSFW"āand can stay in the search within the app. The same could go for "addiction" or similar topics; not traditionally nsfw, but "mature"... er.
Additionally, it might be possible to hide these so tagged stories ONLY in the app. It doesn't really matter for Apple or Google (or it shouldn't) what any host does on the web version of their site as long as they follow the guidelines for the app. It would still be at least half debilitating, since so many users just read via the app and not the web version, but it would be better than nothing.