This one is pretty tough actually. The native slow motion effect is not even remotely feasible for this - it's coded to slow the entire engine timing down, so there's no way to make it "skip" things. With the current stable release, I can't think of anything off the top of my head other than setting a longer edelay to every active entity in the game and also modifying their run/walk speed properties. Or perhaps constantly checking current velocity and reworking it to a given percentage.
Not that complicated at first either way - the tricky part is handling any incoming spawns, and also putting everything back to normal when the effect is over. I would probably set up a hidden entity that constantly runs a loop checking everything on screen and applying the slow motion math above, while also keeping an array of every entity it applied the slow to and what their original properties were. When the entity is destroyed, it runs one last loop that applies the original property values to end slow motion.
DC