If you're attempting to hire a car trailer, you will often find they weigh alot more than what a car trailer for "the normal person" weighs, as the hire companies often put in alot thicker steel, as well as steel in places a normal person wouldn't require it. This is to avoid them breaking all the time when people who hire them once every five years get one and drive them up gutters everywhere, side swipe objects etc etc.
Custom built you can get around the 350 to 400KG mark. That would give you 1600KG if you only register to 2T.
But remember, you than need to work out where your tools and fuel are going...
The full kit for a break away system will set you back only a couple of hundred dollars and is well worth it for a higher capacity trailer so that you can add locked tool boxes to it, as well as tyre rack, and than large fuel container. Some people also find it handy to have a 100L water tank on board, as well as other cool stuff like nitrogen tanks...
Suddenly with an extra 1500KG to legally play with, you can do alot more, and be legal...
Only issue, it's no longer just a matter of any old car with an electric brake system can tow it, purely because they put a stupid light on the dash that tells you if the system is charging or not. Without that light, the RTA will throw the book at you. And yes, they go looking for it!