First, take your ham-to-be and WSM to Antarctica. Seriously, cold smoking is going to be a problem if the ambient outdoor temp is remotely close to your target temp. My WSM gets over 100F when the sun's shining on it during the summer months. I smoked some cheese recently and held it under 70F for a couple hours, but the outdoor temp was about 40F.
My suggestion would be to experiment first. Get the WSM into the shade. Light a couple briquettes, put a chunk of wood on top and see if you can keep the temp low. You'd want just enough heat to get the wood slowly smoking. You'll likely have to add one or two more briquettes every hour or two.
You could potentially put ice in the water pan. If you have room in the freezer, stick the whole pan in there with water in it and let it get good and solid. Short of that, freeze a soda cans filled with water and let those float in the water pan. I've not actually tried this and my concern is it may cause many of the flavor components of the smoke to condense on the side of the pan rather than on the ham where you want them. There's also the possibility of condensation putting out the fire.
I would suggest a briquette snake to allow you to walk away for a longer time but I suspect you'd eventually get too many coals burning at once and it would drive the temp up. Maybe it would work if you laid them end to end rather than shingling them.
The other option would be to use the WSM as the smoke generator and pipe the smoke to another enclosed space. This way the temp of the WSM won't be quite so critical. Some dryer vent hose taped over the top vent on the WSM can be your smoke transport tube. I've done something like this, using a simple cardboard box as my smoke chamber. Cut one hole for the vent hose and another on the opposite side for the smoke to escape. Stick a couple steel rods through both sides of the box to support a grill grate. Might need something a bit sturdier for a ham. I was doing cheese so weight wasn't a huge issue. Maybe just a V rack in an aluminum pan sitting on the bottom of the box. If need be you can try to cool the smoke by submerging part of the vent hose in cold water.
Good luck. I've been making Canadian bacon of late and it's got me thinking about doing a whole ham but I haven't actually done it yet.
