Some manufacturers will weld a stop on to the ends of the upper carriage bar or other have a pre-drilled hole in the face of the carriage into which a bolt is inserted to act as a stop. But OSHA requires any modification to equipment be approved by the manufacturer. You need to discuss this with the local dealer for the brand of forklift you have.

The OSHA requirements for a load back rest is if the load can be lifted above the operators head and there is the possibility the load or parts of the load can fall backwards towards the operator (like when back tilting or driving up a grade) a load backrest shall be fitted to the forklift.

A few manufacturers will approve removal of the LBR if the get a letter of indemnification from the owner & some won't (especially if the forks can travel above the operator's head). The only time I've every see one issued is when the lift was fitted with a "truckers special" mast - max lift height 78" or if the lift trucks was to be fitted with an attachment, like a rotator, carton clamp or rotating paper roll clamp.