A high brow etymological debate on the Reg...how exciting.

Just because there are citations in the OED, it doesn't mean they are correct. They just show current useage of a term/word, whether that be correct or incorrect.

If hoi polloi were to become a single word, ie hoipolloi, or become hyphenated as you had it originally (although that is, as previously accepted, incorrect), then "the" could justifiably be used.

-Simon