This is really nice thing.
I'm afraid, that until devs change things on the server side, items that share graphics like rock, flint, alloys, minerals, iron ore will look the same. It's because they all link to same filename. But this has been proposed few times, without any response.
As for on/off function... i doubt it is possible in-game, although maybe there is a way. Nevertheless it is possible to write simple bash script, that would install/uninstall the mod data. Alternatively, you can use JoneSoft Generic Mod Enabler to install/uninstall mods in any game, that is modable by changing files.
Or you can use custom DataFilesMod.cfg that is included in DataFiles.cfg as i do for switching mods:
### GRAPHIC MODS ###
######################################################
#data\graphic-mods\FootLockerAlert.zip
#data\graphic-mods\Junk Detector 2000.zip #BEEP BEEP
#data\graphic-mods\Junk Detector 2002.zip #Umbrella Triangles
#data\graphic-mods\OESVehicles_v1.zip #Nahrada za auta.
then just uncomment/comment the on you want on/off.