I have one of those and.... i dont believe you can do what you want, Light is used to show if you have voice mail or not. Wont work with actual version thoug(1.4.18), grandstream admited theres a bug with this version.
I've updated my GXP-2000 to version 1.1.4.18 and I see that the configuration for the multi purpose key now include ASTERISK BLF, PRESENCE WATCHER and EVENTLIST BLF. No way for 3CX to support this multi purpose key ?
I think it's very important for a phone system to a simple way to show to the user the status of the line and of the extension also without having a turn on computer with the "call assistant" open