External SVG isn't supported in MAME, and as I said buttons aren't possible with it anyway. You'll have to go for a high res PNG, the same way bezel/cpanel artwork is done for a lot of arcade games. Stiletto manages the MESS artwork website, let him know when you finished it and he can add it there: https://www.mess.org/artwork
I'm praying someday it'll all be lumped in with Mr. Do's collection at http://mrdo.mameworld.info/mame_artwork.php , he does a lot better job of weeding out and replacing the crap artworks with improved versions. We shall see!
