Wow, I live here in Bolivia and I've never heard of this one :o
I'll have to ask my friends in Sucre where the heck you can take this weird bus, at least to take a look at it next time I'm in that city. Although this looks more like something you take outside the city in the countryside. No chance I would be riding this one however, since this is probably hand made and with no warranty :)
Just fyi, there is a decent paved road between Sucre and Potosi, so you don't HAVE to take this bus if you want to get from one place to the other.