I have a weigh-in this week and need to loss 1.6kg (3.5lb) for that. I am thinking of sitting in a sauna, steam room or spa for half hour to an hour just before the weigh-in to get rid of as much of the 1.6kg (3.5lb) as possible.

I know any weight loss will only be sweat and it will be put straight back on as soon as I drink fluids. I just need the weight loss temporarily for when I step onto the scales

What I want to know is whether the time in the steam room, sauna or spa will work to remove some or all of the 1.6kg (3.5lb)... anyone know?

A sauna will definately make you lose some weight temporarily. Fighters and wrestlers do it all the time to make weight. Some need to lose more than that the day of the weigh in. As long as you know it is just temporary water weitght you are losing which I think you do.