I made a quick script to do this (took 1 min to make and test).
To use it, open the map in the editor, then load your map and use the Actors -> Trigger -> RandomTriggerForwarder somewhere in your map and when you next save your map and the script will automatically be embedded in your map next time you save (so no worries about custom packages)
To get it to work you need to trigger it. Every time it is triggered by a regular trigger (or dispatcher or whatever) then there is a chance it will trigger whatever actors have the same tag as the random trigger's event.
You can set the probability, it should be a number between 0 and 1. If it's 0 then the event never occours, if it is 1 then the event always happens. 0.5 means the event will happen half the time.
The example map has one set up that will display a message "Triggered" about half the times you touch the normal trigger.
Plz gimmie credit in the readme if you release a map with it obviously
Edit: forgot to attach file.