I got hold of a portfolio from the 1993 Nintendo Technical Conference which laid out all the specs and workings of the SNES CD system for developers (immediately before they cancelled it).
Their design was somewhat similar to 64DD. Nintendo were going to house the SNES CDs in rugged plastic caddies that contained custom chips for copy prevention and on-board save files. The on-board save files were pretty big at 256 kilobits; equivalent to 4 blocks on a PS1 memory card.
Would have been interesting if Nintendo had launched an optical format 30+ years ago with save capability on the media itself. Might have prompted the industry not to stick with memory cards for the next few generations.