I don't know what the overall weight limit on the NWT is, you can get a fair bit more cargo capacity out of a large saddle bag like a Carradice. I own a Carradice Super C for my Pocket Llama, and load it with up to 10kg, although I am not sure on what its true weight capacity is. Given that it has a 23 litre capacity, its essentially a 5th pannier. I don't know precisely how much more gear above the 70lbs you are looking to carry, but that may be enough. The Carradice bags in particular are very waterproof and of good quality.