Hmm, I don't have a coherent response to the question posed (how to increase online persuasiveness to a level equal to in person), but reading this made me think a few things.

First, I suspect that "online" is too broad of a target for conversations that you want to be persuasive in. What is persuasive in a forum style post may not be persuasive in an instant messenger style conversation. Writing an article that everyone reads and is persuaded by requires a very different skillset than getting the top voted comment on a video.

I suspect you were thinking of a standard text message style conversation, but I'm not certain. If you are looking to influence people through online discussion I'd be surprised if there wasn't an optimal form for this to take, narrowing our target may be of use.

Next, we've had different experiences in terms of in person discussions changing someone's opinion. From what I've seen the use of discussion to persuade the other party to modify their opinion is more or less a mythical act. I give it the same belief as I do hypnosis. It never happens in real life but everyone knows someone who swears by it. Sitting down and telling someone why they are wrong doesn't make them agree with me, it makes them think (realize) that I'm a jerk.

So, given that I don't believe in changing the mind of the other party in a discussion, can we change the bystander's minds? Sure. That's why debates exist, for the most part. So, to return to the initial question, I'd say the first concrete recommendation that I've got is to make certain that your online discussions have spectators. (or rather, make certain no one is under the impression that the discussion is private, given that anything online can be reposted.)

After you decide 'where' to have your discussion, considering both the form of the medium and the number of bystanders, the next most important thing is to pick your partner. In order of importance you probably want a poster who is an advocate of whatever you want the mob to dislike, willing to argue with you, of sufficiently high status to attract bystanders, and a poor advocate for their position.

If you don't mind sock puppetry this is obviously a good use for your drones.

Next up, tone is all important. In a face to face discussion the popular kid wins, you can tell he's winning (and therefore, by backwards logic, still popular) because he's signalling that he's doing so. Bugs Bunny beats Daffy Duck, to use a recent article's example. In person this means using the appropriate body language/tone of voice. Online this means choosing your phrasing. Basically you want to match the tone of the site's most popular users/threads.

So if you are on a Blue/Green blog then use the appropriate mannerisms. (A good way to get a hang on a site's mannerisms is to spend a little while making a program that spits out strings that sound like they come from that site). If you are on an expertise related site then make certain to signal your relevant triumphs. Some sites love the point by point refutation (reply with quote their post, then cut in your answers). On others that's not how its done. Basically you need to have the correct accent-equivalent.

Lastly, there's duration. This is probably the trickiest part of the whole offline-online translation. In person you (the popular/winner debater) can end a discussion in a number of ways, ideally at the moment of maximal impact. Online they can just keep posting. You need a way to gracefully exit after you've maximally impressed the audience. I can't really suggest a generalized way.

Anyway, that's my 2 cents, thanks for reading this long post.