There seems no doubt that "Le Cygne" has Karl Marx connections - It was a Tavern/Cafe when he lived in Brussels. But reports very as to whether he lived there, held Workers Party meetings there or (with Engels) wrote the Communist Manifesto there (In a tavern? Seems unlikely??)!
to a photo of a brass plaque outside the restuarant just mentions a single meeting there on a New Year's Eve - one might have thought they would have made more of it if some/all of the other things were true!! Other photos in the "Marx" section of this photo album site claim the other events but not convincingly!
This site would appear to have been populated by someone who is perhaps more "in the know" and shows the house in which KM actually lived in Brussels to be elsewhere (rue Jean D'Ardenne, 50, Ixelles) and refers to Le Cygne as the place where "where Marx met with his comrades and discussed revolution." I would buy that - it seems a reasonable thing to do in a tavern!http://www.flickr.com/photos/22325431@N05/sets/72157603597357585/
The mention of La Chaux and the Jura watchmaking industry as an example in Das Kapital seems perfectly acceptable for a Connection but we are still 1 short of a threesome!!
My searches of Paris haven't turned up anything - his lodging there at 70 Rue Vanneau is in the Septieme and outside the inscribed area. I think the fact that he wrote about the Paris Commune is a bit too remote! He did meet Engels in the Cafe de la Regence in the Place du Palais Royal but as far as I can make out from the map of the inscrbied area on the UNESCO site the boundary runs along the middle of the Rue de Rivoli - hence the Place is out??
He has lots of London connections of course but none of the more famous is inscribed - I can't even find if he used to spend time at Kew or Greenwich - too tied up in the British Museum no doubt!!