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I've the same problem with my friends : currently the only way is to use demo Version. But I have 12+ people that could be interested in a pbem, and there are only seven playable nations !

Maybe selling keys and downloads is a risky business, although for example Stardock (makers of GalCiv) was successful with that.
However, I can't see any strong argument for not allowing non-registered copies to join Online/LAN games (only). Combat Mission for example allows that (there's no key but CD is required to play).
Given the game "potential audience" and the fact that Online Dom2 games aren't that friendly (how many Dom2 owners play it ?), I can't think of this "robbing" the publisher from paying customers.
People introduced to the game this way will either quit or buy it if they're hooked...