Wouldn't smoke dischargers be meant to cover movements during an engagement and obscure the aime of enemy anti-tank weapons?
Talking of hard to reload, have you ever seen our Ontos?
Its a very small armorered vehicle with four large recoiless rifles mounted to the body.
Aimed and fired from inside but only reloadable from the outside. Still thats four heavy anti-tank rounds that could be fired in less time than reloading a tank main gun even once.
I suspect they had alternative methods of launching smoke grenades should they be unable to climb out, perhaps a GL fired through an open hatch.
PS
I have a collection of photos of the restoration of the Glacier Girl P-38. A friend visited the site where it was being rebuilt.