I know there is a user over at TFK (parent site of bettafish.com) who keeps them. Maybe post over there in the freshwater fish section. I'm sure he'll come.
From what I have heard about piranha, keeping them is similar to keeping goldfish in that they need heavily filtration and large tanks.
From what I understand of characidae (tetra) keeping piranha is basically the same as any other SA tetra. Dim lighting (achieved through lots of floating plants), sandy substrate, and lots of bogwood can make a great biotype in which they will color up nicely and be rather active. In fact, that also sounds similar to a discus biotype setup. Red Bellied Piranha (Pygocentrus nattereri) - Seriously Fish
Water changes on a tank like that would be the same as any other tank: once weekly. You could also look into putting in a sump to increase your water volume because with a messy fish like that, more water means more dilution. When you do get ready to install your dream tank, it might be a good idea to join a piranha-centric forum or a megafish forum. I know monsterfishkeepers.com is a good one.
And lastly, we wanna see pictures when it's done!!