As a user of Smashhax, all you have to do is have no update to load it. Of course Smash isn't going to let you play with your save anymore but when you re-update, it's still there. I don't recommend this exploit since you need a computer and I'm guessing you play the 3ds version of Sm4sh a lot. If you have to use it, get Pokémon Picross and wait to exploit that one so you don't have to hassle with all this.
Edit: As for the demo, the one on the eshop right now is patched so you can't use it through the demo. I didn't see if you have the N3ds model or if you run Linux but just a little FYI, you have to have those in order to use Smashhax in the first place.