Hi Marc,
If you have a wireless (e.g. bluetooth) headphone, you can patch an output to it and add a sound cue that fires at a specific time to that output. You could run a recorded "Fire the X cue" sound message, so the lighting operator knows when to go (and what to do). We have used this approach at several venues where we couldn't plug into their system, and it has worked well (especially if you are ok with minimal human reaction lag).
Best, Vlad