Even when used as a sick tank for small schoolers, they all ended up dying within a day, IME.
I did have a betta in one for almost a year, this was when I was very, very new. Now I wouldn't put one in less than 3 gallons.
BUT, you could do an awesome fishless planted tank. One of those microscapes. I saw a thread here a while back with some really cool ones in even less than 1 g. Some were in like maybe a liter of water.