Please excuse the delay on this response. There was some kind of glitch that it didn't send when I first thought it had!
Hopefully your cabbages are doing alright with this slug onslaught!
In regards to specifically your question of drawing slugs out of your crops and then some "best practices", here are a couple responses from some Alaskan colleagues.
Response 1; "The only thing I've seen draw slugs out of hiding and make them slime their way across concrete is (an expensive German beer which rhymes with fine-i-kin). I had lots of slugs in head lettuce once and applied Sluggo very close to the plants. It did the trick. The client needs to check the label for any 'days to harvest' restrictions. "
Response 2; "SLUGS, the problem that only gets worse over time. I am so sorry to hear that your napa crop is infested. I am assuming that you have the Cooperative Extension Service publication on Slugs http://cespubs.uaf.edu/index.php/download_file/1202/ and are following the recommendations.
Sadly, slugs are very difficult to control. They require many control methods since one alone will not be sufficient. Using Sluggo or any slug bait will prove effective on perhaps 50% of the adult slug population, the other 50% is fine eating your crop. So to answer your question, yes the Sluggo will draw out SOME of the slugs in the napa but not all. Depending on your production methods, you will have to use several control options. Some considered less toxic, some a bit more. Whichever you chose understand that a single method will not be enough, and you will experience some crop loss this year. Also, whatever product you apply, you must READ THE LABEL, it is the law and necessary for good control. Even the less toxic or "organic" controls can have unwanted side effects and so reading the label is required.
While this years crop may be under attack and will experience some loss, my suggestion is to view the long term plan.
First, crop rotation. Slugs really like far to many plants so it can be tricky. Perhaps you can use a small grain rotation, a mustard or buckwheat to till in so they will go elsewhere.
Second, clear the vegetation around the growing area. Keeping a barrier around the plants will help stop access for them and will let you concentrate on getting rid of the ones in the area.
Third, baiting them with either a iron phosphate product like Sluggo or a metaldehyde & carbaryl product like Deadline or a sodium ferric EDTA product like Dr. T's Slug Killer. This will reduce their numbers. No totally, but remember, with a product like Deadline, your then keeping them in your production area since they will not wish to cross it.
Fourth, hand removal. On a large scale it is difficult to think about, however, placing boards or some other flat object they can crawl under when it sunny out is not that difficult. It requires you daily, flipping them over and destroying the slugs found underneath. Either by dropping into a bucket of water till drowned or some other method.
Fifth, after harvest, do NOT leave vegetation in the field. Slugs overwinter under the vegetation and in the soil, there is no way you should supply them an easy spot. Till it if possible and hope for a cold winter without snow cover. A deep frost kills many things, including slugs and their eggs.
While it might seem overwhelming, remember, each slug you kill is destroying 200 future slugs, so use all the methods at your disposal. Many growers release ducks into the production area after harvest to eat up the slugs, but that is in addition to all the other options.
Sixth, don't give up. Learn about slug biology and apply what you know to when they are vulnerable. While crop loss is not desirable, all pests have times where their numbers build up but also times when their numbers dwindle due to changes in the environment, controls, etc. There is no single magic potion or product to get rid of these and if there was would your cabbage still be edible?
Best of luck to you with your production."