Maybe take one half of the triangle and wire it upside down. That way you can make full use of the multiplex, no wasted columns on any of the rows or vice versa. It would add a bit of complication to your code, but you could still deal with it in a way that it wouldn't affect how you make each frame in your spreadsheet.
Maybe there's a smarter, less complicated way to do it... But basically my point is you could fine something to keep from wasting those unused pins...
