Before you get too much further, search out the folks who have tried fire bombers and crop dusters.
Dropping water is virtually invisible from more than a few feet, especially the relatively small amount you will be able to carry.
I tested quite a lot of stuff and found that bulk Splenda is just about right. It is much lighter than stuff like baking power or talcum powder and it is like crystals, which means it reflects sunlight nicely. The only downside is it has to be dry. Since I live in a very dry climate this has not been a problem for me, but, some of you may need to try something else.