This topic is interesting to me as well, as I'm yet to setup my new trailer, and the few things that I need to sort out are the ball weight, and the towball height to keep the trailer level. I've towed with the 4WD a fair bit, and I'm aware of the 10-15% rule, but I'm stunned that having as little as 20 or so KG on the ball can affect a bikes handling that much. Having said that, Honda only recommend 9KG for the top box, which is a lot higher than a towball, but extends beyond the back axle slightly less than a towbar. So maybe there's a mathematical formula based on a few measurements that might give us a maximum ball weight based on the bike not the towbar.
Have you contacted Bandicoot Tony? Just looking at their website now, they only have the Outback, which looks like the old standard, but offer options of spare and 10l carrier. I would imagine that 10l container where it is in relation to the axle, and the spare, where it is mounted in relation to the axle, would be about the same, in terms of balancing towball weight, but I'm just looking at some photos now, and there is bugger all room to pack things behind the axle. I can see what you mean, as it looks to me like the axle is too far back, when considering the cooler tray and the tapered front of the main storage box. I can't imagine that it would be a simple job to move the axle forward, as the chassis seems to taper inward just forward of the front spring mounts, so that would be the last option, but I think it might be simpler to follow Brocks suggestion, and put something on the back on it, so that you can place a variable load behind the axle as far as you can.
I'm no engineer, but the storage area is 1200 long, and doing some rough measuring on the screen, it seems the main area is 800 long, and the tapered front bit 400 long. The axle looks to be about halfway along the main bit, so say 400 from the back and 400 from the front of the 800mm main bit. Forgetting about the tapered bit being a bit smaller in capacity and potentially weight, I'd assume that the axle should be up to 200mm further forward. Take off the slight loss of capacity in the tapered part, but add on a few more KG for the cooler tray, and I'd reckon that 200mm further forward is probably close to the mark.
That 200mm is a lot, and if you were going to compensate for that by putting some weight behind the back of the trailer, I think a 20L jerry can holder is probably the more appropriate size to give a decent range of weights to get it right. 20kg 400mm behind the axle equates to "X" 1650mm in front of the axle?
Maybe we need to workshop this, as I'll be fiddling around in the next few weeks as well!