Quote Originally Posted by Nighthawk View Post
On the idea of a sash, why not a fly plaid and broach?
Some graduation robes/gowns are not purchased by the graduate, just the mortarboard, so nothing can be attached/pinned/stapled or otherwise affixed to the robe itself. Some schools won't allow anything extra beyond the 'approved' Honor Society or other such distinguishing additaments. (My high school wouldn't).

A stole (described by the OP as a sash), if permitted, is probably the best solution in this case. I would suggest a chain pinned to both sides (reminiscent of a watch chain and waiscoat) to keep the stole 'under control' if necessary.