It has a simple XML interface. I think it wouldn't be too hard to have a post/dupe_check page that loads instead of post/show after an upload: query similar, lists them and asks "do you want to keep this upload?", and maybe "do you want to reparent (or reparent + tag dupe) any of these to your upload?". Probably could flesh out the design a little to replace the similar-on-moe link with it (and only show "keep it?" for the initial upload, maybe replaced with "delete it?" for mods). It'd be mostly a regular post index page, with the * on the directlink box replaced with the similarity %.

Saying "no" to "keep it" would just be the same as deleting the image, so it'd still show up in the index in the meantime (without something like a "pending dupe check" post status and a timeout, which would be more intrusive in the code).

API uploads (plugins) wouldn't see this, though.

(I don't know if rq is actively working on dupe checking. There's a "danbooru_image_similarity" module in svn, but it isn't used yet. It's not worth doing our own if he's doing it.)