Directional Blur Node: Progressive Alpha

For stilized motion blur in the compositor, sometimes i'd like to use the directional blur node. Apart from having no visual representation for the pivot location (how about a bitmap location picker?), the thing that bothers me the most at this node is the alpha of the iterations all being the same. I would like to have the option to let the iterations fade out in alpha. Maybe an "Alpha Min" and "Alpha Max" slider?