Sorry, not much help. I've never had one. I don't think they are any more ridiculous to keep than any other freshwater champion giants like Paroon sharks, Arapaimas, Wels, Brachyplatystoma rousseauxii (Dourada), Sturgeon, etc. You are right, only very, very few people can keep these as adults in humane conditions. Anyhow, I'd be interested to hear from the Piraiba keepers too and see their pics.
ECZS (East Coast Zoological Supply; John) sold hundreds of them within 1-2 recent years (~$70; he advertises on H2O-w..ves). There must be tons of juvis and sub-adults around.