Thanks everyone, for your very helpful comments. I've been feeding back your points to the boat builder, so now we're talking about a more bespoke trolley, with no brakes, a meaty point to attach the winch line and a pneumatic jockey wheel (the largest it can be on the leg) - but we have retained bearings on the main wheels. I'm waiting for final details, but I suspect the quote is going to be more than if I hadn't started from here.
All up weight is about 1150Kg, with the force required up the slipway calculated at about 400kgf - and I will be using blocks to align the pull to the centre of the slipway. The winch is rated at 1000Kgf.