You call it an opt-out, but I more see it as an opt-in. Britain as a country wants out of the EU, but this scheme would give those Britons who like the EU an opt-in to sort-of stay in the EU.
I think on the face of it it's a cute idea, and it's certainly a great PR move. "Look, we Europeans aren't the baddies, and we want to welcome those who want to stay".
But I'd like to see the details first. Foremost, how does this reciprocity work? For Britons it's easy: those who want this "associate citizenship" apply and pay for it with an annual fee and get freedom of movement within the EU. But how does the freedom of movement to the UK for EU citizens work?