[SPECIALIST_CITIZEN] - is this hard-coded and unchangeable?

Tourism by Buildings is added based on the BuildingClass rather than Building.
Why? Maybe the Sixth Mystery of the Universe, go ask Neil Tyson.

Part of your issue is you have <TechEnhancedTourism> but no <EnhancedYieldTech> specified. Refer to Eiffel Tower.Though you probably would want to add a <EnhancedYieldTech> to delay when the tourism begins to take effect. Otherwise I think you'd overload the tourism system too early in the game.

You also have both <NeverCapture> and <ConquestProb>.

The real trick seems to be the building must be the default building in its class or it must be a unique building. Though as you were seeing the tourism does not seem to want to stack from having multiple copies of the dummy within the same city. With dummies it isn't necessary to assign them unique to a civ, and for completely hidden buildings it isn't a good idea when you have <GreatWorkCount>-1</GreatWorkCount> to make a <Civilization_BuildingsClassOverrides> for the dummy building. Try completely hiding the building with this still active
and then take a look at the civ in the civilopedia.

Here's the dummy building xml code I used to see the results I got using your lua (which I changed only enough to make it use Rome instead of the civ you had):
I used IGE to add unemployed citizens to Rome the city but all I ever got was the +2 tourism, not +8 like I should have (if everything worked properly the way you would think it ought to) when I made Rome have 4 unemployed citizens. Long and short is that I'm seeing exactly what you are.

LeeS said:

I used IGE to add unemployed citizens to Rome the city but all I ever got was the +2 tourism, not +8 like I should have (if everything worked properly the way you would think it ought to) when I made Rome have 4 unemployed citizens.

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Almost certainly the same issue as this. There is a lot of C++ code in BNW that only detects for the presence of a building with the required effect to add the standard bonus without multiplying by the actual number of buildings in the city (presumably because the base game never adds more than one of any building to a city so Firaxis didn't consider it necessary)

bane_ said:

Tourism by Buildings is added based on the BuildingClass rather than Building.

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Is this incorrect?

bane_ said:

Is this incorrect?

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It's added by the building itself (as opposed to being attached to the class in some way) but multiple copies of the same building within the same city don't have a multiplying effect on the amount of tourism.