I see that Teddy has already answered this through mail, but I'll answer here as well in case anyone else wonders the same:
Unfortunately there's no way to get access to the DRM-free version through a Steam purchase, as it's a Humble feature only! If you regret purchasing through Steam, you could always try to get a refund and, if successful, repurchase the game through our site (or the Humble Store) in order to get the DRM-free copy.
Not sure if this changes anything, but I bought the game via Steam and can run it perfectly fine without Steam by just launching it from where its stored in the Steam folder, works completely like the DRM-free version from what I've seen (IP connections for multiplayer, no drop-ins) even on Frontline builds.
Now the question of "whether or not I could bring it with me" is still up for debate, but other than that it works fine without Steam.
*** Doing this does however cause desync between Steam's cloud save thing and your local saves, and while it is easily fixed (via just uploading whatever save file is most recent) its still worth noting.