Unless I’m missing something obvious this should be simple.
Add the emulated_roku integration to HA and set up an automation to turn on and off the lights.
Add the emulated_roku device to Harmony and set up a sequence rather than command on the play and pause buttons of the activity to do the play or pause function as normal and also trigger whichever button in the emulated_roku automation that you assigned to turn on or off the lights.
I use mine like this to control various lights, a fan and direct select apps on a FireTV Stick.
When you’re in the mood for taking things further, I’d still recommend you look at emulated_roku, you can use your Harmony to trigger literally anything in Home Assistant either as a button press or part of an Activity routine. Was a game changer for me when released a few years back,set up might look a bit daunting but it really isn’t once you actually try it. If you want any help just ask.