Aren't there some weapons that slow the player down without datablocks? Or is that just the knockback?
If you mean certain miniguns, and the OSR, they do change datablocks.
You know how you fall off slopes and move slow while 'scoped' with the OSR? Set the minigame's datablock to a no-jet player, and have an evented brick change you into a horse. Then try to scope with the OSR. Turn the scope off and return to 3rd person view. You are again a no-jet player.
However. There is a way to change the values on a datablock and send that changed data to clients. With this, you could have a separate datablock for each player slot. I do not recommend using transmitDatablocks, as that sends all of them, and displays a message saying it happened. I'm sure there's another function to send individual datablocks to individual clients.