I think endnotes--without any reference to them in the poems themselves--are the least obtrusive way to go.
That way, Maryann and her well-rounded ilk
aren't sent scurrying back and forth from the poems to the endnotes, forced by curiosity's sake to look up references to stuff they already knew. But dilettantes like me can muddle through the whole book as best we can and then--woo hoo!--stumble across the Captain Midnight decoder ring at the end. Everybody's happy. Or at least I'd like to think so.