In the end, I didn’t have to go very far to find a Mac-native Twitter client that was capable of real-time User Streams; there was one already in my Applications folder: Echofon for Mac. I had forgotten that they had opened User Streams up to every user (and not just paid Pro users). After self-updating, User Streams was turned on for all my accounts. Echofon is tiny and sits out of the way — really what I want from a Twitter client (though interface-wise I still think the enigmatic Tweetie for Mac wins). TweetDeck is just too immersive for passive Twitter usage, which is more of my mode these days.