That's strange, yeah it works to add it now. But last time I tried connecting distance map to the volume material, it refused to connect in the material editor. But never mind that, guess my 3ds Max must have been in a strange mood that day. Nonetheless not an optimal solution.
As you say it would be awesome to have some options for the falloff distance & strength so you can easily make the volumes disappear as you would like. Since you have Corona Volume fix/improvements on the Roadmap, this would be a nice addition to that list :)