I love the creamy mouthfeel of UK beers like Guinness, Belhaven, and Boddington Draught in a can. Is it possible as a home brewer to recreate this effect, which I understand is from forced-nitrogenating the beer at the time of canning? Would a bottling spigot rig and a nitrogen gas tank, similar to how forced carbonation can be done for home brewers be possible? Are there any risks/dangers with dealing with compressed nitrogen?
Yes, you can achieve this at home - I often serve these thick-headed beers. In addition to a keg of homebrew, you also need a Stout faucet and a tank of beer gas.
The stout faucet directs the beer through restrictor ring comprising half a dozen tiny holes. This agitates the beer and causes the CO2 to come out of solution as lots of very small bubbles that forms a thick head. Because of the high level of restriction in the faucet, a serving pressure of 30psi is used. The higher pressure gives that beautiful cascade affect you see when the beer is poured.
Beer gas is a mix of carbon dioxide (CO2) and nitrogen (N2) in a ratio of around 25% CO2 / 75% N2. Your gas supplier may blend the gas for you in a tank for use directly. Alternatively, you can get a tank of pure N2 and blend this with CO2 yourself at home in a keg. For a dispensing pressure of 30 psi, first fill the keg to 7.5 psi with CO2, and then fill up to 30 psi with N2. This will give 75/25 ratio, and ensure the beer maintains a low level of carbonation as is typical for these styles.
Strictly speaking, the widget doesn't nitrogenate the beer - the nitrogen mainly stays in the headspace, since it's almost insoluble. What happens is small sphere containing nitrogen is placed at the top of the can, and as pressure builds in the can it pushes some of the liquid into the sphere via small hole, compressing the nitrogen inside the sphere. When the can is opened, the external pressure drops, so the liquid is forced out of the sphere, agitating the beer, forming lots of tiny CO2 bubbles, and a thick head. (If nitrogen were soluble, this wouldn't work since all the nitrogen would have dissolved into the beer and not provide any pressure.)
The stout faucet performs the same job, agitating the beer as it passes through a series of small holes. The role of nitrogen is simply to provide pressure to push the beer, without dissolving any additional gas.
You use CO2 to carbonate the beer first, then serve it under the higher pressure of nitrogen. But you can achieve the same effect more easily. Before Guinness started using nitrogen, all their beers came with a syringe. You used the syringe to suck up some beer and shoot it back into the glass. You got the same swirling effect and the syringe helped knock some carbonation out of the beer. That's what the nitrogen does, too. By serving under high pressure you partially decarbonate the beer, which gives it a smoother mouthfeel.