You can't...
Eyeballs equals the community. Post value could be tied to eyeballs but that would end up with your high quality mailing list posts receiving less value than a post in a thousand post forum thread.
BTW, I will have to take your word on the quality of mailing list posts. I haven't followed a mailing list since the Denizens of Doom (DoD #73).

Perhaps a separation of application developer activities and community activities should be made. Karma for community + Bija for application development.
The value of a contribution can be determined by the particular group that affects it.

The relationship between Karma to Bija can be determined by Nokia for any benefits it wishes to offer at any time it wishes to offer them. In this way Nokia can have the mix that they want at the time time that they want it.

Each groups ego's will be served, heavy contributors to one groups activities and not the other will reap rewards, and those who are closer to Nirvana by having numbers in both categories will receive distinction among their peers in either group.

Just a thought.