For powdered additions, try making a slurry out of it with the water first. If you're going to mix 2 liters of water into your Soylent powder, put half of the water and the flavor powder into the blender and mix it up good first, then add the remaining water and soylent.
The first thread below includes some high-power liquid flavorings, that are advertised as being usable for e-cigarette liquid. Someone was wondering if that'd work, but nobody was brave enough to try it yet.
There were previous flavoring threads, where people were using cocoa powder and cinnamon successfully. (rummage)
Here are two previous ones: