Nothing as small or antiquated as an Aldis-type lamp. Full intensity dual mode green/red deck lighting, to replace/add to the green perimeter lights.
Normal setting is red, when the HLO is happy that his deck is completely ready, he switches them to green.
Helicopter: "request deck availability"
HLO: "Deck available, lights green"
Helicopter: "check lights, we see red"
HLO: "definitely green"
Helicopter: "where are we then?"
Etc etc
Can't be that difficult.