Andy,
Just to throw a cat amongst your already made pigeons.
Extending the bar is a valid but more involved way of fixing your problem!!!

Being a stacer hull the keel line tapers towards the transom such that the rear most centre rollers do not touch the keel. The entire transom is supported by the two longitudinal rails. I would chuck the tinny on the ground, remove the two longitudinals, replace them with 3x2 hardwood planed smooth on the top side which are 500mm longer overal all than what is on at the moment. Bolt them back on with 300mm extending to the rear and effectively 200mm extra to the front. Then slide the winch post back 300mm and the axle bogey 300mm to the rear and your done.!!!! Depending on ball weight you could play with the axle bogey to get your ball weight right. The hull will be supported exactly as it is now, no weding or changing of brake cables required.

For flat beach launching a good technique is to weld a stub axle to a flat plate and clamp that to the draw bar near the winch post. Then put a whole matching hub assembly onto the stub axle. When you need to flat beach launch, put the spare onto the new hub and you have a three wheeled trailer that can be pushed/winched/pulled to the water and dragged out with the vehicle 30m or 300m away if need be. Also you are carrying a full spare hub assembly... much quicker than changing wheel bearings on the side of the highway!!!!

S