I performed a quick Google on the product. Very cool. I didn't dig in far enough to see if they have an API, but I assume there is a way to communicate with this USB device though some kind of API. Assuming that is true, you do have options. What would be even better is if this was an IP device.
In the ideal world, we would build a server-side service that monitors the agent's status (using the 3CX Call Control API) and send an update to the light (via IP). Alternatively, we could create a client-side application that resides in the icon tray, that monitors for changes in the user's presence and sends updates to the USB device.
My point is that we have the ability to receive notifications when a user's presence changes, and I assume we can use that information to update the light.
I would be surprised if this capability was ever possible with the web client, but maybe someone else has an idea of how such an integration could be accomplished with the web client?