Okay after some more digging I found what I was after, some largely undocumented command line arguments.
Adding the following 2 command line arguments:
-UseFixedTimeStep -FPS=30
will guarantee that every world tick function is called with a DeltaSeconds of 1/30, or whatever you specify. Note that the naming of the FPS argument is a bit strange - in no way is it connected with realtime frames per second at all. Without modifying anything else, it will continue to churn out as many frames per second as it can up to the MaxFPS cap. It will simply ignore actual time taken to process frames and pass 1/FPS as the DeltaSeconds of every tick.
See https://docs.unrealengine.com/latest/INT/Programming/Basics/CommandLineArguments/index.html for some other related flags.