I wrote this plugin to augment the gcode that Cura generates. It post-processes the gcode, adding "M73" statements that direct the printer to display the percent-complete while it is printing. I compute the percentage-complete by looking at the total amount of filament the print will extrude, and assuming that if X% of that number has been extruded so far, the print is X% done.

Just download the .py file, drop in it your (CuraInstallation)/plugins directory, and re-start Cura. I hope this is helpful to people.

Note: Thingiverse doesn't allow an upload without a 3d model. Hence I added the smiley face STL. :-)