I can see the value of being able to use previous gcode files by supporting volumetric extrusion, so I'll add it to the work list. I need to work out how it interacts with colour mixing before I implement it. It also interacts with use of relative or absolute extruder coordinates.
EDIT: it also interacts with pause/restore, and extrusion commands in the web interface and PanelDue. There's a lot to think about. Does your slicer generate the M404 command itself?