Unless I'm misreading, I think the OP is specifically asking for a way to use a loadcode compatible version of spider on n3DS for all the neat thigns that were made with that. I don't think I've seen anyone successfully boot spider on an n3DS, so if it is possible, no-one's pulled it off yet.
But, we can use spider when downgrading to 2.1 for OTP retrieval, because downgrading to 2.1 replaces all the n3DS specific titles with o3DS ones. At 2.1 your n3DS essentially IS an o3DS, at least in software, which is why it can run spider.