Have a look at a free blender add-on that features a brush tool for flow map painting. Please note that it supports 2D Image Editor - Paint Mode and also the 3D Viewport - Texture Paint Mode. You can see the tool in action below.
To paint flow maps, you just need to hit the Flowmap Paint Mode Button - this will set your brush into the Flowmap Paint Mode. If you want to exit the mode, just hit the ESC button.