However, in general what currency has a value proportional to its benefit to mankind ? Any fiat currency are going to be out for the same reason than bitcoin is out, any traditional currency based on gold/silver/shells is out too for the same reason as Dutch Tulips: eg. value of gold is recursively determined by its own use as a currency, not by any actual industrial use for example - so its value is as much virtual as a fiat currency, with just a different behavior and control lever.
Other financial asset are more directly related to useful human activities, especially commodities, but at the end of the day they get traded and valued based on their capacity to generate one of the currency above rather than objective benefit to mankind. (eg. Amnesty International "shares" would not be based on its efficiency at improving human rights, but by its capacity to convert it into currency )
The only thing that I can think of, would the slaves as additional useful human being are objectively beneficial to mankind. I guess body part, blood, ... is a less gloomy alternative ?
I'm overthinking this :-)