Good lord, I can only imagine how much work that was... fantastic job!
Stretch goal for digesting the MESS Artwork page: the old Discontinued Decorative Artwork. My understanding through hap is much of the new "external" chessboard artwork that's been made for CBEmu can be ported over to MAME's style with little issue, but you'd have to ask hap for more details, and I am not sure if every chessboard has been done yet.
I am eagerly awaiting the Vectrex work! You're nearing the finish line now!