gvm

In a word..No.

GVM is Gross Vehicle Mass.....Thou shalt not exceed

If the van is plated to 1600Kg, then that is the absolute maximum that that van can weigh when loaded.

Is that 1600kg GVM or GTM?
GVM normally applies to a vehicle - gross vehicle mass.
GTM normally applies to a trailer - gross trailer mass.
I always understod GTM on the trailer as the max mass on the axle(s).

So in theory if you moved some of the load to the ball, you could increase the load you area carrying, since the load on the axle(s) is not increasing.

I would however check the compliance plate for the ATM. Aggregate trailer mass.
ATM is the sum of the GTM (axle load) and ball load.

So if you have 1600kg GTM, that means you already/may have 160 kg ball load (10%), so you only have 140kg to get to 300kg ball load. You need to check your current ball load. Use a set of scales under the trailer hitch.

for reference:

http://www.caravanqld.com.au/media/content/File/VIN%20Plate%20definitions.pdf

If you have 300kg ball weight you 1300kg legal van weight. The ball weight comes off GVM, so reduces what you can carry in the vehicle.

Essentially, the ball weight is part of the 1600 max, so 100kg on the ball counts just like 100L of water in the back. Of course making the ball weight lighter to get more carrying capacity is a BAD idea, as the rig will not handle well at all.