The latest version of DSFix (.8) seems to fix the issues with capturing Dark Souls: Prepare to Die Edition with Procaster. Start the game with Procaster in DESKTOP mode (the Dark Souls launcher will crash otherwise), then once the launcher launches Dark Souls proper alt tab and switch to game mode. Alt-tab back into the game and you should see the overlay. Interactive mode won't work as the overlay's mouse will just stay trapped in the middle of the screen, but otherwise you should be able to record no problem.
Let me know if anyone figures out a way to get interactive mode working, I really want to be able to see the audience chat while in-game.