I decompiled majoras and had a look at the jump. The door frames that are moving back and forth have push triggers parented to them, and under flags "everything(not including physics debris)" is checked. Then when you finish shooting you touch a trigger that toggles it off. Pretty much once the rocket touches the push trigger it goes at that speed, and the door moves slow enough that you can sync them all together. It'd be pretty complicated to implement something like this if you're falling, but other than that it isn't too bad to make.
So to answer your question, no, there is nothing in hammer that I know of *cough* zeq *cough* that slows down time for the player and projectiles, just a push trigger that can make everything inside it move at a set speed.