Is it possible to configure TweetDeck to do native retweets showing "via personname" instead of "via TweetDeck"? I played around in the settings, but couldn't find such a setting.
No, that's not possible. The "via.." comes via the program name to which the oAuth keys are tied to, and these are defined in the twitter API registration page for the application.
All third party applications will always say via the application that made the tweet.
When you do a retweet, you get the following:
If you do
So sorry, you can't do it without, but, as I said, you don't loose anything, it simply says