Would the use of transmission fluid (1 liter once a month to a tank full of diesel) help in keeping the injectors clean?

No, the ATF will only add lubricant to the fuel to prevent wear to the injector barrels in the pump. With later model diesels this is not necessary, they were designed for low sulpher fuel.

If you have an earlier car, one from the eighties or earlier, pour in only a few ounces per tankful. Using inexpensive motor oil would be a better choice than ATF.

It amazes me how ATF has been designated as a cureall for cleaning anything automotive. ATF has evolved so drastically since these old wives tales developed in the sixties, that it can be harmful for many applications. ATF has evolved for use in automatic transmissions only. There are now additives in it that are harmful for many other uses.

Have a great day,

However, it works great for lubricating the throttle linkage ball joints...