I agree that there should be a clear distinction between the weapon used for lethal force and one for less lethal. Many police departments paint the shotguns used for less lethal force options a bright color so everyone knows what it's for.
I suppose one could argue that mounting a less lethal option is no different then carrying OC and an ASP on the duty belt, but I think you are getting into serious liability issues with a setup like FN is suggesting.
"I really meant to fire the OC cannister" Isn't going to cut it when a mistake like that is made. Better to have one officer employ the OC and have him backed by another officer with the lethal force option.
Just my opinion.