The 64DD didn't see a whole lot of releases, and it surely had a limited run of store demos as well. Actually the only one known to exist is a demo version of Doshin the Giant, the game highlight of the 64DD, right next to the hyped f-Zero X Expansion Disk.

The Doshin demo is not identical to the retail version. As seen above not all menu options are available and it seems that you can't save. So when quitting the game the game, it won't save your progress. Also worth noting is that once the day is over, the game will star over.

I've found a video on youtube showing more footage from the demo disk.

The demo disk is pretty rare although at least a handful or two are known to exist and are in the hands of collectors around the globe. With that said, it has been at least some 7-8 since I've see on on ebay. Lately one went for $115 on ebay though.

That's pretty much it, a gem from the disk system Nintendo never really cared for.... but had it been released earlier it could've been an awesome addition to the N64.