The game is up now at http://constantsail.com
The previous version of the game had guilds, and they were pretty active. They will be back soon in the new version, along with port battles where people can work together to seize investments in a port.
I have though of the same thing for this game. If I get enough users, I might run an alternative version of the game where 1000-in-game gold is = 1 BTC, and the game won't create new gold unless people put BTC in. When you do put BTC in, you get 50% for yourself, and 50% goes to the NPC port merchants and such. Everyone would compete for real gold.
I am not sure I would have a cash-out option, as that would require more security features, but I haven't ruled that idea out.