OSHA points the users to the ANSI/ITSDF B56.1 standard. That document, in section 4.17, gives guidelines for the weight of an operator, the type of fall protection that is needed, and the possible impact on the truck capacity.

The actual truck tether anchorage point is tested to a design standard, also in that same standard, which is based on absorbing the energy of a fall. Some types of tethers are designed to prevent the fall where others are designed to stop a fall after it has occurred. Given there are so many factors for the different types of fall protection used and how they interact with the truck, it would be potentially misleading or confusing to communicate a "hard limit" on the truck. This is really a requirement of the user to specify the correct equipment for their application and operators.
Also if you request publication 1089042E page 83 and 84 will advise you.