What plugin does is provides you “MMT Physics Tick” event to run almost any kind of code during physics sub-stepping. It uses the same AddForce() functions, as BP is using, but with extra parameter that is not exposed. That parameter tells if AddForce(), AddTorque() and etc. are called from the physics sub-step or from regular tick. So all this works just as any other physics code works, just with ability to utilize sub-stepping.
Just want to highlight that per-substep updating is active for almost everything that you call from “MMT Physics Tick” event. I’m highlighting almost because practically, code invoked by such event is running in parallel thread, so you can’t call regular draw debug commands or print string (I’ve build separate components for that, which are “thread safe”). Other than that you can do pretty much anything. In some cases this might even improve performance as your “simulation” code runs in a parallel to the game thread (as long as it’s not so heavy that you stall physics thread).
You can mix this with any other code that you have, as long as you are ok with Pawn (I’ll add physics sub-stepping Actor too) having a Pre-Physics tick group, as otherwise you can’t get “custom physics” delegate on c++ side.