All you need to do is to add "cloak:" prefix to entity name, but it works only with CustomAmmoPickup class. Also, only players with sfwcl can see the effect. :)
Here's list of all available prefixes:
frozen:
dynfrozen:
wet:
cloak:
fx: -- for spawning effects
Maybe later I can add support for any classes client-side, but problem is it could be heavy on performance to iterate all the entities in-game to check which one need update and which ones don't :D