I would think it would be difficult to do right, since the barrels on most DWs are not fixed. They couldn't just drill a few holes in the top, because depending on how the customer gaps the gun, they might not line up. Also, there is considerable distance between the inner face of the shroud and the outside of the barrel, so there is a lot of room for fouling to collect.
I have seen some DWs ported. I wish I saved the picture, but there used to be one on Gunsamerica. It had a series of evenly-spaced holes around the circumference of the barrel, and the shroud was ported at the top. I suppose the idea is that the gases will find their way out the top. Personally, I wouldn't want super-hot, super-high-pressure gases eroding the barrel shroud.
Then there is this:
http://www.cz-usa.com/product_detail.php?id=78I have not seen an Alaskan Guide up close, so I do not know how that barrel assembly works. The barrel length is listed at 4 inches, so from the picture I'm guessing that the Alaskan's port system is simply a barrel extension.
Yeah, I'm no fan of porting... Now I'm afraid I'm going to have to utter sacrilege...
To answer your question about how the system works: Porting lowers the pressure in the bore at the point of porting by releasing gas. The bullet therefore does not reach the speed it would out of a "closed" barrel of the same length. Less velocity means less force pushing in the opposite direction. The volume of gas (which has a mass, just like the bullet) produced by the powder also exits at right-angles to the muzzle. Therefore, the recoil force is lowered. The remaining length of barrel provides a little extra mass, thereby absorbing slightly more of the forces. You could get the same effect by loading your ammo to a lower velocity, or by using lighter commercial loads.
The other benefit that porting supposedly provides is that upward-facing ports reduce muzzle jump. Lots of folks swear up and down that it makes a difference. I've never worked out the force vectors, but it's hard to believe that the ejecta of the mass of, say, 20 grains of powder, could have a noticeable effect on the force imparted by a 240 grain bullet, even at near-right-angles to the force. It's a slightly different story in cartridges with much larger charge weight in proportion to bullet weight.
What I do know from standing next to shooters with ported handguns is that the report is noticeably louder, the muzzle flash is right in his line of sight, and as you can imagine, the ports spit sparks and other searing hot junk in directions other than downrange.
Sorry in advance to those of you who may be offended. I just hate porting. Cast lead bullets are proof that God loves us and wants us to be happy. I think that anything that makes a gun unfit to shoot cast lead bullets is an affront to God.