Imparting flavor to steamed vegetables
Water solubility isn't enough - that gets the flavour into the water but says nothing about whether the flavour gets to the food. Consider salt as an extreme example. 

You may get somewhere by putting the source of flavour on top of the food being cooked. There are recipes that do this with mint for example, though they normally add more mint at the end.

The ginger you mentioned may work in a similar way, but testing is difficult as this approach tends to scent the kitchen or even the whole house, so you perceive the flavour even before the food is served.