It's really not a matter of porting. Despite their name, bookshelf speakers really shouldn’t be placed in recessed locations such a bookshelves or an entertainment center, if you want to get the best performance out of them. If you must do this you should pull the speaker as far forward as possible to reduce their interaction with the shelving unit.
As far as the DM600 S3 are concerned, they come with a port plug which B&W calls a bung, which would minimize the possiblity of the rear port interacting with shelving unit you are looking at.
Personally I would take a harder look at the DM601s if they’ll fit. They offer a lower frequency response, and the front port design would reduce the chance of the speaker adversely interacting with the shelving unit you are considering.