When I bought my L-Series, I was told by dealership, 400kg unbraked, and 750kg with brakes. I have these hand written into my owners manual at time of purchase because it is not otherwise listed.
We have a 1986 Cub-o-matic that weighs around 400kg fully loaded. It is not an offroad version, but has enough clearance that it will keep up with the car on any track that the car can still travel with the extra weight behind it. The car will run out of puff, before the trailer runs out of clearance.
Beigewagon.