I've never thought about the in-game reward thing. The problem is, it might encourage spamming and splitting things down into several posts, which means people will drop out because they don't have the time to read everything.
Good idea in principle though. Maybe (at the mod's discretion) pick the most contributive player and let them protect someone (not themselves) for the night?
I also like the idea of rewarding activity, but I don't think that giving powers based on post counts would be the exact route to go. As you pointed out, it may lead to spamming and that is something that turns me off in a mafia game. I just don't like spam in general, so that may have something to do with it, but it also distracts from the discussion the players should be having. Moreover, it's really annoying to have to reread a thread filled with spam.
Now, as I said, I do like the idea of giving players an incentive to post. But it should be based on the quality of the posting, not the amount. That leads to two more questions: What should the reward be and who gets to decide who was the best player. As for the first, I think it would be better if we, at least initially, tried to stay away from giving those players ingame powers. First of all, it will give the mafia even more incentive to kill off the player contributing the most to the game. Not only may the player that contributes the most be able to uncover the mafia, but he is now also more likely to have a power that night. There are also some balancing issues to consider. If we hand out a power to the most valuable player each night, then we have basically created an unkillable pro town power role. If the mafia would also qualify for the role, the games would become incredibly swingy. Now both these issues could somewhat be prevented by making the power relatively weak, but I still believe there may be better ways.
One thing we could try instead, would be to simply have it as an honours. The player who contributes the most gets a special mention in the topic and perhaps we could even make a MVP standings thread, just like the wins/losses thread. If you want to extend it further, you could even have a MVP each day and an overal MVP. Now, this may not solve all issues. Some, especially new, players may feel like they have no chance of winning the MVP anyway, so they may still not try.
As for the way to decide who becomes MVP, there are a few options. What if we gave the decision to the players that get killed. Every pro town player that gets lynched/NKed get to vote who he believes is the MVP. Then in the end the player with the most votes wins. Perhaps this is not all that great, as they will consider the lynch results. (For example, townie A was very active and made a good case against townie B and got him lynched. Townie A now would probably not be voted MVP, simply because he caused a townie to be lynched. But he did contribute to the game, just not to the town). So instead it would probably be better if the mod made the decision. He should be able to decide who contributed the most to keep the game going.
I'm less enthousiastic about increasing the number of powerroles. My main concern is that this does not address the issue at hand. Creating more powerroles may even lead to less active games. Some players tend to lay low if they have a powerrole. Besides, I'm not sure if the games will improve if the increased discussion is simply because more players come out claiming information from having a power role. This wouldn't be so much discussion, as just following the information obtained from night actions. On top of this, it would also lead to more swingy games. If the power roles happen to get lucky the game could be over very quickly.
As for other ideas. I've modded one game where I simply mod killed any player who did not contribute. But that turned out to be a bad idea. Or at least the way it was implemented sucked, I certainly learned that a rigid set of modkilling rules regarding this issue is not desirable. I ended up mod killing one of the more active players, simply because he violated one of the activity laws. (In this case, he didn't vote for two consecutive lynches). I think I like the idea of banning players from future games better. If you, as a mod, feel like player A is not contributing enough, you can make a post warning him that if he does not pick up his activity he will be banned from future games. That should get those players who do really want to play more active. Alternatively, you could also send him a PM if you do not want to influence the game.
So in the end, I believe we should find a way to reward activity (or punish inactivit) without having a (considerable) impact on the game balance.